Punctuation! (or at least a book that explains it)

Can somebody point me to a site that explains English punctuation or, even better, let me know which book would be both concise and comprehensive? I am a native speaker of German teaching native speakers of Spanish, and between us we have so many comma rules in our native languages that I'm going bonkers. (Oh, and on amazon, most books on punctuation are out of print or have no further information, darn it.)

Thanks!

Cornelia

Thanks! They have an interesting site that I'll check out later, and maybe when I'm in the States next time I'll pick it up.

M
(Got an ID now!

I would recommend 'Eats, Shoots and Leaves' by Lynne Truss. Find it on Amazon.
This book has sold very well in England to native English speakers!
As well as describing correct comma usage it also tackles less commonly used punctuation marks such as the semi colon ';'.

The title is better understood when you see the cover which has a panda painting over the comma between eats and shoots. After all, a Panda eats shoots and leaves whereas a hungry, gun-toting gangster eats, shoots and leaves. (He eats first, then he shoots and then he leaves!)