Well, first let me be clear on this: I didn't made the bookX, it's from the zencart user -> Philou.
I just work to a bookshop that runs bookx module and ceon module. So a kind of happy user.
The files that I've sent to git, are actually for myself and who ever wants to take a look a them, cause when zc156 comes along, it's certain that I'll have to make this process all over again, and it's quite boring.
The diference are those that I mentioned. The book product type is no more. There's no download plugin, the last forum post is from 1876 ( more or less )
I think Philou made this module because he needed and was aware that the book product type was not active anymore. Probably that's why he call it bookX , cause you never know the mysteries of live, and the other one, could reborn. That's what I think.
So I would say, that bookx is sucessor of book product_type. It's a filter for books that deals with all those fields belonging to books, isbn, genres, authors , etc, etc...
I'm not sure what ceon module did to the book product type in terms of uri for filters... but I guess nothing.
The same is happening now with those Ceon files for BookX.
It simply recognizes the product type, and adds a uri for the product_bookx_info ( or info display )