Oops, missed out the last parts of your question. Yes, all books are influenced by earlier sources; whether something is derivative or not is often subjective, and depends on what image you want to create of the book, which is what made me think of literary snobbery. The Harry Potter books are chock full of influences (not all concious ones, I think) from mythology and English school stories and other sources besides. And when writers draw on the same traditional themes, it's inevitable that there will be similarities. It's whether you put a new spin on them or fit them together in new ways that counts.