Recommended reading for fiction and reference? This can make life much easier and convenient for GMs and players.

A pretty useful source for ideas, if only because it touches on deities that many others don't. Particularly ancient gods, titans and minor gods. Take a look folks, at the very least it can provide ideas for individual campaigns.

http://www.timelessmyths.com/classical/index.html

Interestingly, I don't have a wealth of specific influences that come to mind for this game. I have said in the past that a handful of works have been greatly influential as I've created this game.

The main influence for the setting is the novel Blood: a Southern Fantasy by Michael Moorcock.

Also influential are the Illiad and the Odyssey, of course. I find Robert Fagles' translations of both works to be particularly outstanding and accessible to modern readers.

For quick information on all things myth, check out www.pantheon.org

Art Nouveau artwork and styling has also been influential. Search on Google for art by Alphonse Mucha and Gustav Klimt.

The League of Extraordinary Gentlemen graphic novels were also very influential. The imagery of steampunk-ish technology therein inspired many ideas, as did the depiction of Mars in the first issue of the second "round" of that graphic title.

I'm sure I'm forgetting some other influences. Oh! I just thought of the Spelljammer d20 supplement that appeared in Dungeon/Polyhedron last year -- that was an impetus for me finally putting together some loose ideas I had back then. Anyway, these should give some indication where Nine Worlds "came from."

www.greatdreams.com/jason/jason.htm - Jason and the Argonauts. Another tale that might be useful to GM's and players.

www.uoregon.edu/~joelja/odyssey.html - the Odyssey online.

www.classics.mit.edu/Homer/iliad.html - the Illiad online.

From games, GURPS Greece by John F. Ziegler (sadly out of print, but check the shelves you never know) gives an excellent overview of the setting and the myths and how to game with them and in their style.

For GMs and players planning on working extensively with Atlantis, there is GURPS Atlantis, by Phil Masters, and GURPS Places of Mystery, by Phil Masters and Alison Brooks. The first gives a approachable overview of the classic Atlantis and how to use the sunken one in a variety of games, while the second gives a capsule overview and additional material on the mysterious places of the Mediterranean.

And while they're not classical myth, the author freely admits that he was inspired by (and occasionally stole from) the classics.
Cross the Stars by David Drake which retells the Odyssey in SF terms.
The Voyage also by David Drake which retells the Argonautica in SF terms. While not the source material, they are inspired by it and may prove useful to GMs and players alike.