welcome to the Hook Productivity Forum, @pl0. According to a PIM thread , I think Hook is working with Mathematica with the exception that on some occasions the Hook window needs to be invoked twice in some circumstances to avoid the “No linkable item in” message. Is that what you are seeing?
They do work for me as of this writing.
yes I can’t get it to work with Mathematica  at all.
just to confirm, I’m using Catalina, Mathematica 12, Hook 1.3.3. After two or three invocations of shift-command-space the link is finally copied. It Works!
There is FWIW, draft coverage of Kindle in the
Copy As Markdown Link Keyboard Maestro macro at the link below.
The Kindle macOS app doesn’t yield its book titles, locations, and ASIN codes very willingly to scripts, so the macro is in the tradition of Rube Goldberg / Heath Robinson GUI scripting at best, but you are very welcome to test it. Seems to be working here on Catalina 10.15.7 with Kindle.app 1.30, but no guarantees.
Since this original post, the list of Linkable Mac Apps has grown considerably. New and updated integrations are discussed in the Releases category.
We understand eagerness to support Kindle. However, that script is, again, quite brittle. We advise Hook users to avoid relying on it. At a minimum, they should build some redundancy. In Cognitive Productivity books I described principles for that. (Those book contain cognitive science and principles derived from that for meta-effectiveness.)
We appreciated your early contributions to this forum, @RobTrew . We reviewed several of them and were happy when required to edit them–that is fine, we are happy to help. However, we do not vet solutions on other web sites. By systematically referring on this forum to solutions that are on another website, as you have been doing lately, you are making recommendations to our community that we cannot vet [and that the community in our experience is less likely to vet ] which puts others in an awkward position with respect to trying them out. Best to post on this forum specific solutions tailored to the framework this community is actually using. We certainly don’t mind cross linking, but this is to be born in mind. (E.g., I was invited by the developer of Bookends to keep his community abreast of Bookends related Hook info, and that is vice versa.)
In any event, we at CogSci Apps revisit Kindle app from time to time, now particularly with respect infrastructures we’ve been developing.