This one may be very silly but how do familiars work? I call it, it appears in my inventory and then? What am I supposed to do next to release it?

In ToEE they work like the charms of Diablo II. If the familiar is in your inventory, you get the bonus. They don't die, or leave or anything else. They just sit there taking up space. Think of them like a rabbit's foot, but you get the whole rabbit! ;)

And what is the bonus exactly? Is there a difference based upon what familiar you get?

There is a list in the manual on page 66 that tells what all of them do for you.

best familiar is rat... +2 to fort save... perfect for a wizard.. most familiar give a bonus to a skill... then rat gives +2 to fort, and another gives +2 to reflex which is on par with the fort...

OH, and they also have the role playing value, there are ALOT of things in the game that have no effects, like the clothes you wear etc... having a familiar is cool for a wizard... it stays with you, it has its own name... it gives you a slight bonus... by the end of the game you would think of it as a part of your wizard/sorcerer

If you don't have a rogue or bard in your party for the Appraise skill, give your wizard or sorcerer the Raven familiar, which gives a +3 Appraise. That li'l +3 boost can mean tens of thousands of gold in the long run.

If you worship platinum, you can make a L1 Wizard/Sorcerer mixed with nine levels of rogue or bard, for the highest possible Appraise. You'll swim in more cash than Scrooge McDuck.