master index of multiple book

[Milton, Cynthia]

I concur. In my last job I did exactly that (so engineers could find the doc they needed easily). The uber-book contained over 300 separate docs, and another 400-page big book as a sub-book. 

When you generate the index, make sure you DO NOT UPDATE ANY NUMBERING, as otherwise everything gets screwed up. To differentiate index entries for the individual docs and the big book, I added a meaningful prefix to the numbering, so the big book entries were H-<pageno> and the individual doc entries were W-<work instruction no>.

I used normal index markers from the big book and Subject markers from the individual docs, using the otherwise-non-functional Volume number.

/Cynthia