Question about leveling up spell research

[Griffy]

After playing a druid for years and years I decided to try a bit of the dark side and rolled a little Necro. I am really enjoying playing it. Since I have now gotten to level 20 I'm noticing more of our spells "the pets in particular" require research.

Does anyone have a link to a good guide about where to find the pages we need? Is it always strictly random mob drops?

Does anyone have any resource tips or tricks they wouldn't mind sharing with a neophyte?

Thanks in advance

Griffy