I encountered on the research tree for magic the gaining of the book of enchantment, the book of summoning, then a higher level of a book of combat (not sure of name but added combat spells and strategic attack spells like earthquake), but haven't seen how you get elements you don't start with. Might be in quests.

Just like SuperTimo, I got that Morrigan (or something similar) book from a quest and didn't see any additional spells available. Not fully implemented feature yet?

Indeed, you can get some additional books by researching magic tech tree as well. However, you should also be able to get some from the quests and more could be added via mods. At least that is what I remember by reading some official posts during late beta.