Can you give your follower a spell book and they will learn it? Because I was gonna give Serana the soul trap spell book.

No.

That svcks. Is there any other way?

Im sure theres a mod for it if you play on PC. Each follower has his/her arsenal of weapons, armor and spells.

http://www.gamesas.com/topic/1470739-ck-to-turn-housecarl-from-caster-into-melee/

I was trying to do something slightly different, but you can look at the other posters tips cause they will help you out. Also go to youtube and search for "best in slot skyrim creation kit" and check out episode 7 and maybe episode 1. Episode 7 is how to create a new follower, but if you pay attention to the video he is showing you the directories for where the 'actor' files are. You just have to navigate it and find Serana and modify her spell list.