Ask friends or family about other books that remind them of your book. Often they will tell you things that you did not think about. Make sure that you read the books they may suggest because you might find that they are irrelevant.
Check out books from award lists that match themes or the genre of your own book. There is a reason these books have won awards, and having a look at these is definitely well worth the effort. Websites such as Amazon.com provide significant resources through lists of acclaimed books.
Read recommendations of books in high-quality newspapers. Ensure that any books you may choose are relevant to your book and that you are not just choosing them because they are popular or have been recently published; do not be afraid to recommend lesser-known or older texts.
Look at publisher's websites for inspiration, bearing in mind that they are likely to be biased. Many feature lists of recommended titles and online communities where members give you honest opinions about books that they have read. Choose a publisher that is suitable for the genre of your book or relevant to the themes it contains.
Go to your local library or bookstore. Most will frequently produce updated lists of recommended titles or librarian's recommendations. If they do not, ask a librarian or member of staff for suitable recommendations.