mstdn-ebooks and FediBooks have both been updated to handle a breaking change in Pleroma's outbox format.
This change actually happened quite a while ago: https://git.pleroma.social/pleroma/pleroma/-/blob/841e4e4d835b8d1cecb33102356ca045571ef1fc/CHANGELOG.md#107-2019-09-26
It seems that nobody noticed that any Pleroma instances running 1.0.7 or newer didn't work with mstdn-ebooks or FediBooks, but today someone *did* notice, and I've fixed the issue.
This is a solvable issue, but there isn't a solution that is as easy to implement or as well functioning as the Mastodon/Misskey method. The best I can think of is "remembering" that there are still 10 posts to download, and next time mstdn-ebooks runs, downloading those 10. This has its own drawbacks, however, and is a pretty major change to implement for the sake of a single platform not implementing this particular part of the ActivityPub spec. I'd like to note that the 'prev' key is optional in the spec - Pleroma isn't failing to comply with the AP spec, just not implementing a particular optional feature that saves a lot of headaches.
This issue puts it succinctly: https://git.pleroma.social/pleroma/pleroma/issues/751
"No 'prev' property is defined, which could logically point to the 'future' page - this is a the main problem [...] the only way for this check is to request initial endpoint page [...] that results in unnecessary duplicated requests and data transfers, and complicated syncing logic."
@email@example.com's anti-chud pro-skub instance for funtimes