Typically book titles are ideally italicized or otherwise underlined. For example:
In his book Some Book Title, John Doe describes...
Is there any style convention for what to do if you are limited to unformatted plain text (i.e. italics and underlining are not available)?
No formatting? All that leaves you is quotes:
In his book "Some Book Title", John Doe describes...
Be sure to set the title off with commas. You can use quotes, but capitalization alone could be enough to make clear that the words are a title:
In his book, Let Us Now Praise Famous Men, James Agee describes the lives of southern sharecroppers.
However, the is a ultimately a style question. Each publication, organization or association will have a specific rule about the formatting to be applied to book titles.
To clarify on capitalization, rule for capitalization is that you capitalize the first letter of the first word and all significant words save for conjunctions (and, but, or, ect.) and articles (the, a/an). Take the following title:
"The Prince and the Woman He Loved"
You'll note that the word "the" occurs twice. In the first instance, it is capitalized as it is the first word in the title, while the second instance does not capitalize it.
When ordering the book Alphabetically, the first significant word is used, so it would go in "P" for "prince" not "T" For "the" and would typically be written "Prince and the Woman He Loved, The". Typically, this rule only applies to articles as leading conjunctions would be alphabetically arranged by the first word. Citations may opt for a significant word, however.
