Should a title of a book be underlined?

Yes if you are following the MLA rules from the Modern Language Association style manual, also known as the MLA Handbook for Writers of Research Papers. These rules are usually followed in research papers in the areas of the humanities. (Note: Please excuse me for not underlining the title. This answer box does not have an underlining feature.)

No, if you are following the APA rules from the American Psychological Association style from the APA Publication Manual. The 5th edition of this manual states that titles should be cited in italics. Prior editions stated that titles should be underlined. The APA rules are generally followed in research papers on scientific topics.

These are rules for very formal research papers. For less formal writing where no specific rules must be adhered to, it is probably best to underline the title.