Try Google Search. Your first step is the easy-to-use and ever-present Google search engine. Just go to Google.com and enter in as much as you know about the book. Try entering character names, keywords and anything else you know about the book, such as the title or publisher. Google will produce results most relevant to your request.
Use Google Books Advanced Search. If Google the search engine didn't find the results you want, you can get more targeted results by using Google Books. Google has literally millions of books scanned and entered into its database. Use the advanced search options to search by keyword, ISSN, ISBN, publisher and even publishing date. You can search books in other languages as well.
Search your public library. You can head down to your public library and search its catalog in person, head to its website to search online or use the WorldCat library search engine (see Resources) to search more than 10,000 libraries. Once you find your book, it will tell you the closest library where you can check it out.
Ask your librarian. Librarians get these types of requests all the time. Many of them can probably tell you what you are looking for or at least help you find it in the stacks.
Ask someone online. There are many question-and-answer sites, like Yahoo! Answers and Amazon's Askville (see Resources). You can post your question and hopefully a reader will be able to provide you with an answer. Other good places to post your question are in reading forums on websites like Goodreads, Barnes & Nobles or Borders.