1. I always used some Divinity Wiki
to look things up.
The site has some useful other info too.
2. There's a difference between Classic and Enhanced Edition. In Classic, you could learn 3/5/7/9/unlimited spells for the corresponding ability levels 1/2/3/4/5.
The Enhanced Edition introduced the Novice/Adept/Master system which has different number of spells and adds another restriction. This could be called the 'spell tier'.
The Abilities page
on the wiki has a good overview for the Enhanced Edition, for Classic, it's trivial as written above.
There's also some page on the wiki that shows the AP penalty you get when your ability level is too low. (e.g. you are able to learn any novice spell with ability level 1, but you get an AP penalty if the spell is spell level 5 or above, because ability level 1 corresponds with spell levels 1-4, ability level 2 with spell levels 5-8, etc. up to ability level 5. There is no spell in the game that has a spell level above 20 and I'm not even sure if there's a single one with L20 even.
With the 'any' above, I do *not* mean that you can learn every novice spell at the same time with ability level 1, you can have 3 novice spells, but you can choose any of the novice spells for these 3.