Today's library essential is pretty much a library unto itself, but easily "owned" by every reader here. As you can see, I am talking about Andrew Lang's famous collection of 12 books in his Colored Fairy Book series. I won't list them all here, just provide covers and links to print versions from Dover at Amazon. I own all of these books and they sit within easy reach on my shelf despite their ready availability on the internet.
This series was multicultural a century before that word was used so imperatively. It also--even a hundred years later--has tales that haven't been translated into English anywhere else. Then there are the wonderful illustrations by Henry Justice Ford and G. P. Jacomb Hood.
Then there's the secret knowlege that much of this series was translated and edited by women despite Andrew Lang's name on the cover. He was the big name and had brand recognition so his name is on the cover, but his wife and other women were the primary editors and translators. He often acknowledges them as such in his introductions.
The colored fairy books were the introduction to fairy tales for several generations and are still fairly well known today. I see random references to them in other books--just this past week a character in a book I was skimming kept referring to the Green Fairy Book.
But in the end, the treasure trove of hundreds of tales from around the world gathered into this small library is a wonder and an essential in my library. You can see the old editions on Google Books, even download PDFs of the scans, or find editions on Gutenberg, or if you need a quick list and sorting capabilities of the entire library, I recommend the collection at Mythfolklore.net. The illustrations aren't there, but you can sort the text by author, country, book, etc.
So that's an easy addition to your own folklore library. Makes up for some of the more expensive titles I'll still be featuring this month.