Hey
I know we'll never likely know the source texts that Roberta, Jane Jensen, Mark Seirbert and Lorelei Shannon got their knowledge of all the most obscure mythologies and fabled creatures, especially since they worked in an age (for the most part) before the internet, but I was wondering what some good books on learning about Mythology, Fables and Spirituality (Mysticism, New Age, etc) would be?
Wikipedia and your local library are good places to start.
Hit the library first. If you're looking for an excellent collection of them, Ebay will occasionally have an old collection of books called "The Young Folks' Shelf of Books" that were published between the 30's and early 60's. Granny had a stash that the twin and I read voraciously, and they've got some interesting, obscure ones. D'aulaires book of Greek myths started my sister off on that front, and it's beautifully illustrated. The Bullfinch's version is considered more definitive, but it isn't an easy read.
Drat. I should ask my twin. She's the librarian...
Agreed on the library and wikipedia. I think in this case, wikipedia is a great resource for finding out about the less popular stories and creatures, etc, and can often give you sources to go check out for more information. I've done plenty of quick research for roleplaying games I've run on wikipedia--for use in a computer game, I'd say doing more in a more reliable resource would be a good idea on top of that.
If you'd like a web site, Encyclopedia Mythica (http://www.pantheon.org) pretty much covers all the bases and more.
Here is one which might be the source of the Nightmare seen in KQ6.
http://books.google.co.uk/books?id=WeX82Hhy2cQC&pg=PA245&dq=nightmare+female+pegasus&hl=en&ei=zRVgTO2sDKL9sQa454S-Bw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDUQ6AEwAA#v=onepage&q=nightmare&f=false