There's Carl Phelpstead's excellent Tolkien and Wales where he dedicates three chapters to language, including a reasonably long section for the connection between Welsh and Sindarin (in its various incarnations). And of course from there you may check the literature list (which includes, I can see, Jim Allen's Introduction to Elvish as well as essays / papers in various collections). Mark Hooker's Tolkien and Welsh is also rumoured to address the phonological connections between Sindarin and Welsh (as well as quite a lot of other stuff). And of course there is Tolkien's essay ‘A Secret Vice’ in which he discusses his own language-making hobby.
Troels Forchhammer, physicist, Denmark
Things done and over and made into part of the great tales are different. Why, even Gollum might be good in a tale