When does a book have good scholarship?

If a book is still relevant after 10 to 15 years of being originally published, then that pretty much is indicative of the book's scholarship. Usually, because of advancing scholarship, a book is revised every 10 or more years; though some are revised after only 5 years, but that is usually when a Hardcover is published into Softcover or something like that (sometimes there's revisions because of poor proofreading but is not totally indicative of its worth).