This book (and when I say "this book," I mean the entire trilogy) is an extremely intricate and powerful book that rewards multiple readings. To call it a good "adventure book" or to note that modern fantasy literature is virtually all just sincere flattery of Tolkien or to observe that Tolkien's own precedents were epic and mythological literature or to wonder at the depth, consistency, beauty and, above all, REALITY of Tolkien's Middle Earth is to damn with faint praise. Read it again. There is always something more.
What struck me during my most recent reading was the deeply Christian ethos of the book. The ring is, of course, the power to indulge one's pride, the capacity for ultimate self-aggrandizement. The heroes of the west are all great because they are all able to deny themselves that power: Bilbo, Gandalf, Aragorn, Galadriel, even Boromir (after a moment's slip) are all great because they are humble. There's a wonderful passage in Mordor where Sam is wearing the ring and sees himself as the irresistible gardener, striding through the wasteland with a flaming sword and using the power of the ring to restore the land. It's delightful characterization and powerful writing.