I think it depends on the book. When I really enjoy a book I look up the author on the internet, I suppose to understand more of the context of the book. I usually read a lot of the classics, so they usually have a good, long introduction, which can be helpful. (I always wait until I've finished the book first to read it, though.XD)
I tend not to read too deeply into books that don't seem to have any symbolism in them, but when I do notice some parallels or symbolism, I like to think about it, and how well it contributes to the overall feel of a book.
I think symbolism has to be used sparingly and thoughtfully to maintain impact. Repetition of the symbol or having the character's relationships stand as symbolism are the most interesting ways of using symbolism in my view. I think an overuse of symbolism can make a book contrived, so the amount I look into it also depends on the quality of the symbolism itself.
I like books that are left completely to interpretation in theory, but usually when I read them, they drive me crazy! XD