I have read "The Adventures of Tom Sawyer" and "Lolita."
Lolita is a mad genius piece of art. I can only imagine how long Nabokov took to create that novel. Every single sentence is painstakingly crafted and tells a very wonderful story. It's funny, and heart breaking, and funny, and a very worthwhile read. If you do pick it up, do note that there is a Foreword in the novel that is actually a part of the novel. I don't know about you, but I tend to skip Forewords and Introductions since a lot of times they tend to be critiques and the like, but yeah...don't skip.
Tom Sawyer is a good, fun, kids book. It's really not meant to be much more than that though. I love Twain, but I like him because his humor really strikes a chord at lashing out against the hypocrisy of racism and race and the like. Tom Sawyer doesn't do that really. That isn't to say it isn't an enjoyable read though.