Here are some interesting Tolkien links that I am aware of. I will
possibly update this in time but I really do not know when or if.
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.
Estate (added 03 March 2020):
A wonderful resource with lots of writings, paintings and other magical
things; the link is their sitemap.