A good GM is always learning-so get your learn on with these:
From the Spirit of the Century (OGL SRD)
http://www.faterpg.com/dl/sotc-srd.html#tips-and-tricks
Also this thread (I believe listed in a Paleo post but still good for a quick glance for the list:
Gaming Books that Offer Unexpectedly Good GMing Advice?
http://www.treasuretables.org/2007/10/gaming-books-that-offer-unexpectedly-good-gming-advice
