"His book is essential for understanding prehistory." "The prehistory" is specific (ie. Jura's suggestion "the prehistory of..."). Without "the" we mean prehistory in general. To get back on track of your original question, "What's this book *for*?" "It's for understanding prehistory. In fact, his book is essential for understanding prehistory."

first one is correct.

His book is essential for understanding the prehistory of........... [ you need to finish the sentence] OR His book is essential for understanding the prehistorical aspects/ previous history of .......... OR Example: His book is essential for understanding the previous history of the American Indians.