No, not about thread necromancy 
This ritual permits the summoning of ghosts. So far so good.
Does the dead person already have to have/be a ghost? or is a ghost 'created'?
Are there any ways to block this spell? Perhaps if a given ghost has already been destroyed (eg using Lay to Rest the Haunting Spirit) or imprissoned via some spell or effect (there are a few available).
What other limitations are there?
Please help a would-be necromancer out 
If it was my saga, these would be my answers...
The spell could call up the spirit of any dead person who has not recieved a proper burial or gone directly to heaven. They don't have to have a "pre-existing" ghost... or rather, by being unburied, they do have a ghost even if it isn't wandering around active.
The spell could be blocked by a number of things, including but not limited to magic resistance, the spirit being bound or trapped in some way, and specific magical effects designed to prevent the spirit from being summoned.
Other limitations I would put on summoning the dead have less to do with the spell and more to do with the dead. The spell, after all, in no way grants control or even communication with the summoned ghost. Also, most summoned ghosts would probably be unhappy about being summoned (imagine how well disposed you would be to someone who forcably dragged you away from your daily existance).
I'd say most corpses would leave a ghost (naturally only if they haven't been buried with a Church ceremony, are saints etc.). But somebody without unfinished business or who've died a peaceful death of natural causes might not leave a ghost behind.
Although most times when Player magi need to speak with a ghost, it is of somebody who has died violently and suddenly, as the result of a crime or accident, and most certainly have a beef with being dead.
But why, why on earth is this a Ritual? It is 'just' an 8th magnituyde spell and uses no Duration or Target demanding it be a ritual. It's not even a permanent ritual.
This is why I would allow it to 'create' a ghost where one has not naturally occurred. So it still can't affect those who have had a Divine burial, or a body whose ghost has already been destroyed by magic (such as Ghost's Eternal Oblivion), but it can raise a ghost from a corpse where a ghost would not 'naturally' occur, as well as summon to the magus's presence a ghost which has spontaneously arisen due to some unfinished business. This makes it a bit like a more impressive Whispers Through the Black Gate, that also leaves a ghost behind once the magus has finished.
Mark
If this is the case, I'd like a 'light version' of the spell, which summons a naturally occuring ghost (which one could subsequently cast Coerce the Spirits of the Night on), which shouldn't be a ritual. And if IopStD can summon a ghost not naturally occurring from the place where the person died or usign an AC from a body part, the design notes should note it is a Ritual because of this.
Frankly, I HR it to not be a Ritual anyway - it doesn't do anything which requires Ritual status - summoning via arcane connection is purely mechanical for Hermetic Magic - spirits are just slightly easier to affect than lumps of human bodies. As for Whispers Through The Black Gate, I've made that a partially integrated nonHermetic spell in my sagas, requiring a lab text to invent, since what it does is nice and flavourful, but also contrary to the standard practises of Hermetic Theory.
As has been noted before, the rules for anything to do with spirits in Ars Magica are complex and contradictory at the best of times, so house ruling 'til you're satisfied is the on;y sensible way to go.
The True Name question and the Soul Limit make it a "Not totally" Hewrmetic base effect. Since most ghost are Mentem based, and really arent't too complex you could summon it with the Guideline base 15, summon a Mentem spirit. But The spirit must be a existing ghost, probably with another limit from the spell design.
..and how do I bind it into my service?
There's an example grog in Covenants which is a bound ghost.
how to make something like that?
That would probably be a different spell. You might want to check out the rules for Hermetic Spirit Magic in Mysteries Revised.
Well yes, obviously.
I was hoping I could get a suggestion in this thread though
Fair point... for whatever it's worth, here are my thoughts...
Controling a ghost is Coerce the Spirits of the Night (rego mentem 20). Easiest way I can think of to have a bound ghostly grog would be to create a magic item with this effect. Have the item maintain concentration effectively giving it continuous duration and use it to command the ghost to perform a specific task like guard this passageway, or sound the alarm if you detect intruders. Ideally, one could enchant the dead grog's own skull for this effect giving both a shape/material bonus and a penetration bonus and providing a body part to summon the ghost using Incanation of Summoning the Dead.
Also, you get the lovely image of a wizard surrounded by the skulls of dead servants, each one binding a ghost to him.
So... the duration merely means that the ghost is forced into the presence of the casting magus for that amount of time? Or?
Otherwise it'd have to be momentary, neh?
The duration of a summoning spell is the time that the creature is forced to remain in the summoned location, unable to try to leave. Of course, you need another spell (Coerce the Spirits of the Night being the core book example) to compel other activities. A short duration on the summoning spell gives you the chance to cast other spells before the critter departs - the summoning can be envisaged as a command to "Be here."
Or, simply cast the ritual within a circle of Warding. Then the ghost stays until you have cast Coerce the Spirits of the Night.
BTW with this definition I might even start using this spell with my Tremere necromancer, instead of only Voices fron Hollow Spaces. But it sure is expensive in vis.