Your quest log should have a list of where each book is and the quest helper arrow should point you to the right boss.
Here they are just in case you may still need the information:
Life History-Lady Blackhope on Unicorn Way Fire History-Alicane Swiftarrow in Firecat Alley Myth History-General Akilles on Cyclops Lane Storm History-Harvest Lord on Triton Avenue Ice History-Prince Gobblestone on Colossus Boulevard Death History-Lord Nightshade in Storm Drain Tower in Haunted Cave
