I know I've said this already, but I want to mention the Index again for emphasis.
It may seem like a small thing, but it isn't. Knowing where the find information and intuitive placement of information is important (even outside of rpg books). Maybe I'm just getting older, but my perception is that a lot of newer books (both rpg and otherwise) do not effectively use (or sometimes don't use at all) basic components of writing a book: such as an index, glossary, or table of contents.
5E indeces could be greatly improved -especially the PHB and especially with newer books with tweaked information.
Additionally, a pdf file to give a brief rundown of changes from PHB 5E to PHB 5.5 would be helpful.