If you're fluent in Rust, you will have no problem learning C considered as a language. If C has any faults it is that it is too simple. Rust even sensitizes you nicely to the worst pokey bits of C, so you won't be wandering blind through the landscape of memory safety like most people learning C. I don't think you'd need books or anything, just grab some tutorials online and start banging away. It's not a hard language.

I say "C considered as a language", because in my opinion and experience the real challenge in learning C is C considered as a platform, that is, learning how to build C projects, how to actually pull in dependencies, how to resolve them when they blow up, how to build cross-platform, understanding the several toolsets used by real projects to accomplish these tasks (raw make, autotools, cmake, etc.), dealing with library conflicts, etc. C is a nice little language, too simple for 2022 and something I wish people would really stop using, but it is at least simple. The C platform is a clusterfuck nightmare disaster zone, if not multiple independent clusterfuck nightmare disaster zones.

I can say with a straight face I "know C", inasmuch as I know the language and have even done some non-trivial work in it. (Not much, but some.) I have never in my career learned C as a platform. This is a choice, and I still think it was even a good choice, but what I'm really highlighting in this post is that this is certainly something that is noticeable, that the fact that I "know C" has actually been of almost no utility whatsoever when I encounter problems and have to fix them because the problems are almost always in the platform. I have made do with a very cobbled-together understanding of the situation and a high degree of "google-fu" on this topic (beyond just "pop the error message in verbatim and hope for the best") and I've chosen not to pursue this any further.

So I would recommend to you that you consider your goals here; do you want to "know C" as a language or a platform? Both are fine options but they lead to different solutions.