I love euphemisms. By disguising negativity with flowery prose, many people get distracted from the message at hand. Here are some translations of the statements in this letter to layman terms:
I'm afraid Grove/Atlantic is a small publishing house and we have to be selective about what we take on. (Translation: "We don't have the kind of budget to take on a project that we think will fail.")
Due to limited resources of time and manpower, we cannot accept unsolicited manuscripts or manuscripts that do not come to us through a literary agent. (Translation: "If you're not good enough to have someone take care of these things for you, we're not interested.")
As you may already know, there are several books on the market that can be useful for beginning an agent search. (Translation: "Obviously, you're going to need a lot of help to actually find someone willing to represent you.")
Thank you for thinking of us, and good luck with the project. (Translation: "Please never contact us again.")
Now don't you feel better?