Resources for learning to write wrappers to C libraries
Kliment: With Python, there is a well-defined and well-documented C API, as well as support for ctypes. Ctypes allows you to call into C-interfaced libs with very little code. The Python C API allows you to write modules in C that behave like modules in Python. With Scheme, there are multiple implementations and no standard one, but most implementations provide a functionality for calling external code. Here's a nice list: http://community.schemewiki.org/?ffi
Rather than tutorials, I'd start by trying to dissect existing library wrappers and figuring out how they work. This is how I learned to write Python extensions.