hey dude welcome to cumbo
The text book that I most recommend getting is the 'Anatomy and Physiology' book by Seeley, Stephens and Tate. If your struggling with anatomy then you may want to invest in the atlas or human movement book.. but the Seeley text book is pretty comprehensive.
Don't waste your money on the psych text book by Gerrig and Zimbardo.. biggest waste of money ever!.. mines still in the original plastic. Pretty much buy textbooks for subjects that you are struggling in or need the most help in. If you can afford them why not buy them all
cheers