I've just run into the same problem and I've coded a simple command line HTML concatenator, so you simply issue
HTMLCat book.htm part1.htm part2.htm part3.htm ...
at a command prompt and it merges part1, part2, part3, etc. in the specified order into book.htm.
Alternatively, if all your filenames are in alphabetical order you can simplify and do things like
HTMLCat book.htm cover.htm chapter*.htm
The thing is still a bit crude (it simply keeps the HTML <head> of the first file and concatenates all of the <body> contents of every file after that, so all files must use the same encoding, no ID attributes are checked for duplicity, etc.) but otherwise fully functional.
Anyone interested? It's writen in REXX, so it can be run on Windows, Linux and pretty much everything else right away with
Regina REXX