A link that I always use when I want to browse, is

http://www.sacred-texts.com/neu/celt/index.htm

There is a wealth of books and text translations here for any (ancient) civilization you can think of, from the translations of folklore and myths (Irish, Scottish etc.) to Scottish faerietales, Native American religion, mythology and folklore, Icelandic lore (Eddas and Sagas) and so much more. I hope you'll have just as much fun here as I do :)