Top left corner, there should be 4 boxes for books and a separate 5th one to the right for tactics manuals (leave blank).
Click on a box and slot the books in.
If you are a new player, some words of advice:
1. Save your fuel, don't buy keys and don't trade yet. You want to save for store books. If you don't plan on raiding, get White Flag (WF) first and then save for Union City Shortcuts (UCS) or Ammo Con (AC). UCS can be avoided if you want to spend $30 for DMU which will shorten return times (DMU = Deathmobile Upgrade)
2. Focus your leader's points on fighter, at least 2/3s or a full fighter spec, other points go to medic.
3. Spending real life money, first things you want to buy are the deathmobile upgrade and the inventory upgrade (especially important later on)
Good luck, and I hope you enjoy the game. Feel free to ask any more questions.