The Hero with a Thousand Faces (wiki link here). This is the book to read if you want to get some interesting perspectives on the commonality of mythology and stories throughout cultures. The author's thesis is that much of mythology follows the same basic story, which he calls the "monomyth". It is comprised, basically, of the following story, quoted from the book:
A hero ventures forth from the world of common day into a region of supernatural wonder: fabulous forces are there encountered and a decisive victory is won: the hero comes back from this mysterious adventure with the power to bestow boons on his fellow man.
Obviously the book goes into much more detail and studies the mythologies of many cultures and how the hero journeys in said mythologies. Basically required reading for myth making :-)
In addition to Campbell's monomyth, take a look also at the heroic paradigm. You can see this in action in a wide range of heroic literature from kids' stories to serious adult myth & legendary literature.
In addition to Silmarillion, I'd also recommend you look into the Letters of JRR Tolkien and perhaps Carpenter's biography. You can glean quite a lot of his thinking and process through those sources.