Are you talking about InDesign CS5 or InDesign 5, which would be CS3?
I suppose CS5 as IDML did not exist in CS3.
More important is the question which OS do you use?
Do you have some conflicting styles? Would it help to put first all documents into an INDB, synchronize styles and masters and after that moving the pages into a single document?
Yes you're right, it's InD CS5. I'm working in 10.11.2. I'm fairly confident that the styles aren't conflicting (mainly because it was a designer who who set them up, not me but I'll check through. I don't know INDB but I'll check this out too. Thanks
Ups, are you aware that CS5 does not support El Capitan an it can cause crashes.
Maybe that a book itself works for you fine.
I'm certainly aware that it crashes, tho it's been doing this for longer than I've been working on El Capitan. No one has said that to me explicitly before, so thanks useful knowledge. I can just put the files into CS4 as that seems less delicate. Not sure what your last sentence means.
You don't have to have all your pages in one document in order to have a book that can share styles, index, TOC and coordinated page numbering. An InDesign book file ties the individual documents together. This allows you to work on a smaller section at a time, but keep all the advantages of a single document.
The gray book Dialog lists all the documents involved. They stay separate, but key elements like Styles, Master pages, colors and numbering can be synchronized.