The Fellowship Extended DvD has alot of interesting special features dealing with how Tolkien came up with his ideas for the books. You might look into borrowing/renting it if you're interested... I think the dvd has about 2 hours of specials on how/why he wrote, so its more than worthwhile if you're looking for information.
And Debalic is right, Tolkien was upset with the lack of truly British mythology, so he decided to construct his own by pulling ideas from many other cultures.