One of the biggest things I would find useful is a consolidated list of base effects for each art combination, along with at least one example spell per base.
Extended discussion on things like wards and the like, with concise rulings on how circle targets work, etc.
A very clear and easy-to-read 'build a spell' rules set - with some handy tables showing the different ranges, how many magnitudes they add, etc. Include in this magnitude-boosting guide anything that is specific to certain forms (e.g. terram).
Consolidated lists of ranges, durations and targets. Better rules for things like size-changing spells and how to work out just how much the size changes by (I worked this out myself, but only by inference).
There's a lot of general magic rules that could do with a nice, concise, easy-to-read set of rules for both the new player and the old player alike. As long as such a tome included a combination of clarifications and a few new Hermetic things I think it'd be a worthwhile tome. I know more than a few people in my group would buy it - and these are people who already have all the HoH and RoP books.
A single reference point for Hermetic magic would be nice. Its all well and good having house-specific stuff in the house books, but part of the problem is many of the house books contain stuff that isn't house-specific. The Jerbiton chapter has a big block on imaginem that isn't Jerbiton-specific, the Merinita chapter has a section on house mysteries that are labelled 'Not For Merinita!', the Flambeau chapter has a very good chapter on general use of magic in combat. You need a score of at least 2 in 'Ars Magica Book Lore' to even know where to find half the rules - I personally think a consolidated Hermetic Magic book like this would make an excellent companion to the core rulebook for new groups without treading on the toes of the other books out there.
I'd also include a chapter on 'how to do certain things with Hermetic magic.' A chapter on how to make an effective necromancer with some good spell suggestions, on how to make an effective fireball-slinger with notes on which skills are important and why, etc. Ars Magica is a complex game, and one thing it really lacks is something to help new players get into the game comfortably. Such a chapter could also include some notes on spell combinations and long-term thinking : things you can do with summoning storms, making crops grow, causing a person to decide to wear green each time they wake up in the morning.
The idea isn't to replace the imagination but to give it a boost. The most dangerous thing you can give a new player is a blank slate - most people get intimidated and back off.
Throw in a couple of adventure hooks and ideas related to certain spells in the sidebars and you've got a winner.