I want to learn MSX-C, where do I start?

If you'll use any up-to-date compiler (like SDCC you already mentioned), any ANSI-C book will do the job. You only have to be careful if you're using any CP/M C compiler, since they're older than ANSI-C specification.

I feel a bit guilty about advertising my own stuff, but you can start with the MSX-C tutorials I've been writing:

Relearning MSX

I haven't been writing for a while, but there are 18 chapters. That will be enough to keep you busy until I continue posting new chapters (soon).

SDCC compiles fast because it runs on a PC. However, it's not the easiest to set up, and the code it generates isn't the fastest because it always passes arguments to assembler functions via the stack.

By comparison, MSX-C compiles slower (since it runs on the MSX), and it has other limitations such as the size of the source file, which can be handled easily once you know how the compiler works. Check the tutorials on my site for more info on this.

I recommend MSX-C over SDCC, even in spite of the complications, because it generates faster code. MSX-C is actually an MSX version of LSI-C, and it always tries to pass as many parameters as possible via CPU registers, resulting in faster programs.

Something else that you will have to keep in mind is that MSX-C was released before the C language was standardized as ANSI-C, which is very close to modern C. The MSX-C compiler understands K&R C, which has slight differences with ANSI-C. Not a big issue.

In MSX-C there is a compiler directive that forces the code generator to pass the parameters in the stack, apart from a couple other changes in the way it handles integers:

#pragma pdp11mode

The SDCC compiler doesn't have any switch or compiler directive to force parameter passing via registers, that I know of. However, I'm not an expert in SDCC and I may be mistaken. Better let someone else who knows it better confirm whether I'm right or not.

Look here http://sdcc.sourceforge.net/mediawiki/index.php/Z80_code_size