There are several reasons that compel people to buy books and as an author understanding these reasons helps you shape your work accordingly.

Entertainment: This is especially true for fiction books. We buy such books purely to be entertained by these imaginary characters and their stories. There are also a number of nonfiction books bought for entertainment purposes. 

Information: They say learning never stops. Many people read countless books purely for information. The world is always evolving and we need to keep ourselves informed of these changes without forgetting the past. Many books fulfill this purpose.  

Required Reading: Not all books are bought by choice. Sometimes it is because of book club picks or textbooks assigned in school. This factor does not directly play into a reader's choice but authors can still utilize it by making sure their books get into the hands of those making such reading decisions.

Escaping Reality: For some, reading, especially fiction, is a way to escape reality. Well-written stories have a way of drawing the reader into the story, helping them forget about the real world and its endless problems. Your book should, therefore, be able to capture the reader's mind and take them on a journey into that fictitious world.

Celebrity Authors: For many celebrity authors, the aim of writing is often to share a part of their world with their fans. Much as fans are always ready to buy their favorite celebrities' books, celebrities must not take this for granted. Respect your brand and your audience by giving them a book worth reading. Whether you are writing a biography, cookbook or self-help book, make sure it is informative and engaging.

Inspiration: Many people read to be inspired. Whether fiction or nonfiction, readers love books that show them there is a greater purpose in life and a better life out there. More importantly, they love books that give them hope that they too can find their purpose or live that life. Readers need to feel a connection with your characters and/or message. They should be able to imagine your heroes living in the real world and feel that your message is applicable to their day-to-day life.

Impulse: Just like with any other product, many people make unplanned book purchases. The key here is to make sure these spontaneous buyers see your book. Make sure the book is accessible to the impulsive buyer and that the title, cover, and summary are captivating enough to capture their attention.

Established Authors. Surveys have shown that most people buy books of authors they already know and love. This is why it is important for authors to create a solid fan base by constantly giving them quality books.

Trend: Sometimes people have no bigger reason to buy a particular book except for the fact that it is the trendy thing to do. Certain subjects became popular for a certain period and buying such books feels like the right thing to do.  

Recommendation:  Many people will buy a book because someone they trust recommended it. Potential readers will value the recommendation of someone they know and trust above any form of marketing.

As an author, make sure your book fulfills their needs.