This one leaps out for me as a book, and as a volume that is central both to Lowell's work and to the whole of 20th century poetry.
For Lowell, it stands at a point of balance between indulgence and discipline. The formal strictures of his earlier work were loosened, but not yet abandoned. The personal material was gushing forth--raw and agonizing and vital--but still being shaped by aesthetic choices, still being focused and crafted and refined (in a way that wasn't always the case later on).
Of course it influenced many who followed (perhaps too much), inaugurating the confessional movement and opening up a new range of possibilities in terms of subject, tone, and style.
But most of all, for me, it just makes for a thrilling read: when I return to it, it excites me and inspires me to try to write again, to dig back into the worst moments of pain and vulnerability, and press down ruthlessly, and listen hard, and watch, and wait, and trust that something fresh will emerge...like a child dabbing her cheeks to start me shaving, or a mother skunk with her column of kittens, who drops her ostrich tail and will not scare.
I'm sure everybody has a copy already, but if not:
http://www.amazon.com/Life-Studies-U.../dp/0374530963