but they are all compiled and I cannot download them separately. The whole series is over 10Gbs and thats too much for me.
You can download a part of a torrent.
When you load a torrent in your torrent app, e.g. uTorrent, every file in the torrent has a priority, default "normal". High priority is gotten first.
You can set priority "don't download" then those files will not be downloaded.
So select all of them, (control-A) and mark them "don't download". Then select the book you want and make its priority "normal".
Also, there is a single book torrent for that book here:
https://audiobookbay.lu/audio-books/the-last-continent-discworld-book-22-discworld-rincewind-book-6-terry-pratchett/but it isn't narrated by Nigel Planer.