What is the best resource for learning (N)ASM?

Here's an interesting page from a former teacher. http://drpaulcarter.com/pcasm/

I haven't read through his online book, though.

I have tried a few assemblers, and I found FASM is the most beginner-friendly. It has a somewhat bad macro language, but otherwise it is an excellent piece of software. It supports both Windows and Linux object formats (either 32 or 64 bit). It has an active and good community, and plenty of beginner-friendly examples.

For my Assembly language class, we're using the book that Bill recommends. My professor also recommended this book: Assembly Language Step-by-step: Programming with DOS and Linux. He said that this is the only book that he knows of that covers NASM (and it's a tad outdated).