Is there anywhere on the Internet (or elsewhere) where I could find some kind of family tree so that I can understand how all the various publishers, divisions, and imprints are related to one another? (For example, I know because my agent told me that Berkley and NAL are related, and I was able to discover that they are both part of Penguin, but I can't figure out how St Martin's and Tor are related, even though I know they are.)
well, they're cousins sort of. Here's an explanation for Tor.
I'm not sure if a "family tree" exists for the other publishers. I'll bet if it's out there, one of the snarklings will know!