It seems like, you might have more luck with books than with trying to piece things together from bits and pieces on websites - I possibly don't know any books about Sumerian mythology, sorry, but searching your local library or alibris.com (because Amazon are crooks) might get you something that'll give you a general idea of what archaeologists know about Sumer and its mythology, and give you somewhere to start... Source: 12 months ago

I buy used books at alibris.com, but, apparently, they work with sellers (20 USD per annum). I takes an investigation to check out those. Source: about 1 year ago

Highly recommend checking out alibris.com as well. Source: about 1 year ago

I collect first edition / first printings of rare science fiction from the 60s/70s/80s. I've found abebooks.com and alibris.com to both be excellent when trying to find obscure pieces. Source: about 1 year ago

Drive-by comment to recommend http://alibris.com as a great used book, music, dvd source. Source: about 1 year ago

Occasionally, I'll visit sites like http://BookBub.com and https://centslessbooks.com/. Source: about 1 year ago

One thing I use frequently but often fail to mention is CentslessBooks for free Kindle ebooks. Some are better than others, but you can always find plenty of classics, good scifi, and mystery. Source: about 1 year ago