It's more of an example than a resource, but the 'git2' crate that wraps libgit2 does a good job of providing a nice rust interface over a C library.

I think maybe the ZeroMQ rust crate as a case study is something that is non-trivial: https://github.com/erickt/rust-zmq

My co-author Carol Nichols gave a few talks on this a while back, but there's not a ton of resources, it's true. I would look at big libraries and see what they do. I know it's not the best thing, but it's probably the best that exists right now.