Personally, understanding a book has nothing to do with how much I enjoy it. I have read and loved many books that left me puzzled or feeling I only vaguely understood some things. On the other hand, the books that I feel like throwing against the wall (hey, that's something that's more difficult to do with an e-book - not that I would do it in either case
) are the ones where everything is laid out in detail, especially the character's thoughts or feelings, or even what I should be feeling (that's the worst). I need some blanks to fill with my own imagination, and I do think some measure of mystery or ambiguity adds to the enjoyment.
And to more or less tie this with the original question, I tried to read Tom Sawyer recently and felt it was rather lacking in that department (mystery/ambiguity). I didn't finish it.