Well a tome is a book. Summae/tractatus are gameplay categories, but a tome is an item.
So that spell would work... and only it if you want something definitive.
How to obtain that spell?
some mercere magi or legati/mercantes from other covenants may have it and cast it if you offer something, like a copy of that book if you have the right (by luck, Mercere being dead, you should not have any trouble)
there may be a market for that kind of spell, like there is for spells of covenant creation against payment.
You should also consider doing a simple copy of the book (see Transforming Mythic Europe for photocopy spells&items). If no one knows it, you have got yourself a readable copy.
I don't think any spell can return lost information, such as that on damaged books. The details about which information is ruined and which can be repaired magically aren't addressed, to my knowledge, anywhere. To keep things simple and maintain game balance, I'd suggest that in general it's impossible to repair books with magic. I'd only allow such repair under special circumstnaces - with a Ritual that uses an Arcane Connetion to the writer's ghost, for example, to InMe the information out of him. And even that's a stretch.
For game balance that's a fair point, but it begs the question of how damaged a thing is before it is no longer able to be repaired with creo magic.
I.e. How badly damaged does a sword or table need to be before it cannot be restored? Are the engravings on these items restored? Say a special engraving on the blade which has been removed by damage.
That book basically needs to be totally ruined for it not to be able to be restored (the book is dead). If magic can heal a body back to health from Incap and multiple heavy wounds, then a book should be recovered.
A table is a table is a table.
But there's no Art for information (no, not Intelligo either).
If you know kinda-sorta what the table looked like, you can recreate it. You could even use Intelligo to query the broken and battered table what it looked like, and re-create it from that. It might not be a perfect recreation, but it would be very close.
But a book doesn't know the information it contains, so you just re-create the contents on the book unless you know those contents by heart, or have available a copy to examine.
Creating, or re-creating, a blank book is nearly trivial, though probably a waste of Vis, but casting a spell and suddently you have en intact book, complete with its original content, is outside the realm of possibility with Hermetic Magic. Arguably, this relates to the Hermetic Limit of Time.
I suppose it depends on how the book has been damaged. Magic can certainly help with water damage. If it's been ripped to pieces, then presumably it can be painstakingly reassembled in the way that paper shredder output can be pasted together. Magic should be able to help with this too.
If pages are just gone, I don't think Hermetic Magic can help. If the book is really important and you're willing to go far far away, Socotran Aloe Magic can regrow lost parts of objects. It could make for a good story.
"Any magus can create bread or cloth, but in order to create an elaborate mosaic depicting the foundation of the Order of Hermes the caster would need to know what it should depict. If he was wrong, the mosaic would also be wrong. Similar considerations apply to repairing artificial things."
Fair enough. So if a character has both their legs blown off and a wizard repairs the damage they get that wizard's closest approximation on the legs. Kind of funny.
"Where did my scar go" or "hey I wasn't this hairy before!
Damage generally makes the information harder to obtain, rather than destroying it. This can be in a number of ways- the image may be blurred with water damage for example, or it may be partially destroyed in a way that can be recreated- for example fire damage may remove or damage some of the words or letters to where they cannot be read directly, but may be inferred: "Reseting your P_rma at sunset" for example is fairly obvious despite the missing letters.
For magic to restore either of these (or create a more legible copy) would seem to me to fit under craft magic (whether creo or rego) rather than trying to use magic to create information, but may require the magus to spend a season reading it before it can be restored... (which if it is a tractus only restores the quality for those who read it after you).
Non-magical, but practical: Check with the Great Library of Durenmar to see if they have a copy. If they do, use the two to generate a correct version. This is not a violation of a Calf and Cow principle (even if Durenmar would be concerned); you have a damaged copy already and so long as you pay for the costs of copying, binding, and so on they should be cooperative. This is a work by a Founder - such things should be in circulation. If Durenmar does not have this, or is lazy about responding, inquire of Harco. They may have a copy or be able to locate another copy, and might be interested in covering some of the costs of perpetuating their Founder's magical work.
A book could be missing gatherings or have torn pages that reduce it's quality, and Creo spells are fairly useless for repairing these, as magic has no way to recover lost writings. A book would be an Arcane Connection to lost gatherings, but not forever, so if the gathering was lost a few decades ago, it's of no help (assuming the gathering was not destroyed).
It's possible that a very old book of magic might have its own spirit, especially if the book was in strong magical auras for centuries. Such a spirit would likely have complete knowledge of the book's contents.
It's possible that Soqotran Aloe magic could completely 'regrow' missing parts of a book. This seems to be a special feature of Soqotran Aloe magic.
Durenmar is the go-to for checking for existing copies, though Harco might also have one, given the author.
So i did follow your advise and went to Durenmar to have an expertise on it, it is a lost tractatus of Mercere about Magic theory, i tried to trade it for some free study time in the Library of Durenmar the only thing i was offered was 1 season there, i feel a bit swindled ... The one who expertized it was the chief librarian of Durenmar, he was telling me that the Mercantes would never allow that.
Now im hesitating as a Bonisagus, should i contact Harco and if yes what should i ask ( knowing that this Sg is know to be stingy.)
It's a bit different situation, than taking a book which has 10 missing pages, and doing Creo Animal to get the pages back with all the information contained upon them. Creating the thing is easily accomplished with Hermetic magic, but, there is no Art for recreating knowledge from the aether. That being said, getting access to the pristine original, and the damaged book, I'd definitely allow some craft magic/creo rituals to restore said book, but that's likely not worth it, in the long run.
You are now veering into saga specific territory. We can't really advise you on how to proceed, and even if we did, a stingy SG is a stingy SG. Take what you can get from the tractatus and move on would be my course of action. Unsatisfactory, I know, but little we say can really help you with a stingy SG.
I would take a different stance than that of many people on this thread, and say that based on the principle of contagion, one can argue that Creo magic should be able to repair a damaged book, even if a bunch of its pages have been destroyed. I do not think reasoning about "information" fits the Ars Magica paradigm. Basically, the book was a metaphysically whole object immediately after having been written (which incidentally it wasn't when it was only half written) so you are restoring it to that "wholeness". The argument that you could not create it from scratch does not apply (it would apply, however, if you tried to Creo back the full book from an undamaged, partial copy of the damaged book). My aesthetic sense tells me that glosses would probably not be recovered in the "healing", but that just may come from the fact that I hate when people write over/underline/etc. books.
I would tend to support Ezzelino, especially for a book that has been bound with resonant materials. At this stage, one can argue that the physical book is also contributing to the quality of the information, thus there is a strong bond between the physical form and its content. So it should be possible to restore to full quality with the right magnitude. Taking Creo Corpus guidelines and deciding if a few pages missing are a light wound for a summae, but possibly a medium or a heavy wound for a tractatus. Damage by fire or being completely soaked in water might be considered as aging crisis.
If people are concerned about possible abuse of such spells, making them rituals, and using Creo Corpus guideline should make such use rather expensive and only worth for exceptional work. It has potential as story hooks if PCs are trying to reconstitute the diary of a dead Quaesitor to find out the truth about Diedne or Tytalus corruption. Some spells might be invented to utterly destroy a book or a written text beyond repairs to erase any trace or compromising proof.