• Encyclopaedia of Arda: This is a huge resource of a lot of different types of informations, maps, essays and many other things in the Legendarium. The creator of this website has read much more than I have as far as what's available to us Tolkienists.
• Online Tolkien bookshop: Besides books by Tolkien there are also many others related to his works as well as some unrelated to the Legendarium or even Tolkien's works at all - like that of his grandson Simon.
• The Tolkien Estate (added 03 March 2020): A wonderful resource with lots of writings, paintings and other magical things; the link is their sitemap.