My best guess about the "600 pages" rule is that such a length gives the book a nice, thick spine, and a nice, thick spine is a great place to plant a catchy ti

My best guess about the "600 pages" rule is that such a length gives the book a nice, thick spine, and a nice, thick spine is a great place to plant a catchy title in enormous lettering. That probably helped when selling the book on shelves. Furthermore, length could be used as a proxy for comprehensiveness, which helped when selling the book through B2B channels, like corporations, academia, etc. (Oftentimes, such buyers were more likely to complain about what a book didn't contain than what it did, hence, the drive toward kitchen-sinkism). All of which was great for sales back in the day, but not so great for the quality of the books.