Understanding the book, in the way Analytical reading state it, is about having the help of intrisic aid. We also can have extrinsic aid from other books, web, encyclopediae, person, videos… But this shouldn’t be done synchronously.
Reading abstract or extrinsic comment on a book should be made only after we read it.
From: How To Read A Book, The Classic Guide To Intelligent Reading - Mortimer J. Adler, Charles Van Doren