Just make sure you use a throw-away email address... Sitepoint will spam you like crazy. No such thing as a free lunch :-)
My experience has been that they're on the low-volume end of the "we have your email" spectrum. Not nothing, but not hideous.
Rather generious formatting, though. Printed, this would be one of those books that could've been done with half the pages and still be as readable. (And for online reading, who needs margins and reference sections?)