Your favorite book is almost certainly chosen from the 129 million books that Google knows about: http://www.fastcompany.com/1678254/how-many-books-are-there-...
That gives you 27 bits of entropy.
The average book length is probably not over 400 pages. An average page probably doesn't have over 25 sentences on it. So the whole book contains only ten thousand sentences.
That gives you 14 more bits of entropy.
The total is 41 bits of entropy. This is one-eighth as secure as a 4-gram composed of random words from a corpus of 2k, if we measure strictly by entropy.
The situation is actually much worse, though: your favorite book is probably a popular book. So the number of bits of entropy provided by the choice of book might be a lot smaller than 27. I would guess that it's perhaps 10.
And many of those 129 million books are not very different. They contain quotes from other books, reprinted short stories, folk tales, set phrases, and so on.
In practice I think it might be difficult to mount a password-guessing attack using the Google Books corpus, because it's hard to get access to that corpus. The Project Gutenberg corpus would not be so hard.
