Why can't I find this ebook in any catalogue (even Am&*n)

Updating my list of ebooks borrowed from the library (Norfolk Library Service, not available as a source), but I have signally failed to find any entry for The Complete Mackenzie Collection, although it is here, so should be found by Overcat, no?

Only if somebody else found it in a library source. Try Bloomington (Il) public library. WorldCat says they have it. Assuming that works, it would then be available for future use. Amazon and manual entries are not in Overcat.

For what it's worth. This work contains entries for 1 - 4 and 1 - 5. WorldCat seems to indicate that it is the 1 - 5 edition in that library.

Strange that 35 members must have added it manually! I did try Amazon (UK and US) when I couldn't find it elsewhere, but those failed too.

I'm not sure which edition I read, so I'll remove it and have another go using the source you have kindly researched.

In general, major public libraries are the best sources for ebooks. Toronto, San Francisco, Brooklyn and Seattle Public Library are my go to for ebooks. Also CARLI, especially if it's non-fiction.

The existing entries of your book need not all be manual. If they were imported from Amazon, they wouldn't show in Overcat

And just because it's not in Amazon now doesn't mean it never was.