Mind you, there are several books in the game. One is the one you buy from Sebastian which then appears in your laboratory, the other one is on your desk in your bedroom. The latter you need for learning new spells.
For learning about protective barriers, you need to get past the ghost girl at the tower and meet Melissa. Shield spell is crucial here.
https://f95zone.to/threads/warlock-and-boobs-v0-503-0-1-boobsgames.18692/post-14086012. At lvl 5, you should be able to learn it, provided Jill is in town. Also potions from Rose can be a huge help.
Can't tell you anything more now. If you post a save, I could have a look and try to see what's going on in your game. No promises though.