"Sometimes some of these little side excursions are useful and I manage to fit them in the book somewhere"

This quote by Jack Vance speaks to the importance of putting in the time to explore ideas and ideas that may not be directly related to the main topic of a book. He suggests that these side adventures can be helpful and can be included into the book in some way. This could indicate that the side trips provide extra context or details that can be used to boost the main subject. It could likewise imply that the side trips provide an unique point of view or insight that can be utilized to further establish the main topic. In either case, Jack Vance is suggesting that making the effort to check out these side trips can be helpful and can be used to enrich the book.