What did the book contain? The past tense might be used when talking about a book previously read.
Check the Table of Contents. A book might contain a preface and an interesting introduction, a certain number of chapters with intriguing chapter headings. A book might contain a good index and a comprehensive bibliography. There might be various illustrations, some special photos, charts, and maps. A library catalog record generally provides a list of what a book contains.
Now do you have some idea of what a book might contain?
In a detective story, perhaps the book was smuggled into the jail and, several pages cut to form a secret storage space, contained a gun. Of course, the gun was discovered, so the book no longer contains the gun. The sentence is past tense. :-)