A selkie is a being that is caught somewhere between a human and a seal. They are shape shifters. Usually you will only see one as a seal but every so often one will take off it’s seal skin for a time on land. This is usually what spawns the stories about them, for if a person can capture the hide of a selkie, that selkie will be under their power.
Selkies are faeries from predominately Scotland and Ireland.
A good site with a little bit of information is http://home.cogeco.ca/~eligio/selkies/selkies.html. it goes over some basics in a fair bit of detail.
http://www.orkneyjar.com/folklore/selkiefolk/ is another ok site and has just some basic information on orkney legend, a place on the northern most edge of Scotland.
I’ve also mentioned selkies before in the film The Secret of Roan Inish https://faeriesight.wordpress.com/2010/05/18/the-secret-of-roan-inish/ that is a phenomenal movie about selkies and this is the scene from the movie where the selkie legend is told. Parts 2 and 3 are on youtube
There are countless books on the subject, but a good one I have read is called Tales of the Seal People by Duncan Williamson. It is a collection of stories Williamson compiled when he went to Scotland.
Another good book is called People of the Sea by David Thomson. Again another collection as this guy goes around Scotland trying to find selkies.
There are also many newer fiction books that feature or have selkie cameos. A book that features a selkie is Seven Tears in the Sea by Terri Farley and a couple of books with selkie cameos are The Serpent’s Shadow by Mercedes Lackey and the last book in the sevenwaters trilogy by Juliet Marillier, Child of the Prophecy. All fantasy books of course.