The Lovely Bones. Lerner spent sixteen years working as an editor before moving into agenting, and from what I've read about her, she was legendary as an editor.
Thankfully, Betsy Lerner passes on a huge amount of information through her blog, and in her book The Forest for the Trees. In the book, Lerner shares the perspective of editors in the publication process, and gives tips on making your queries and submissions stronger.
She also offers advice on handling rejection, dealing with ediors, and what to do if you experience writer's block. The book was published ten years ago, but Lerner is working on revising the information for the current world of publishing, and the new version will be released in fall 2010.
Lerner's blog is a humorous peek into the life of a literary agent, with its associated victories and travails. Her irreverent blog sometimes reads like a memoir, but because of that, it's fascinating. A selection of blog posts I like:
The ups and downs of publishing.
What happens when an author really revises.
Lerner's own query letter for a new project.
The importance of a good title.
The website of the Dunow, Carlson & Lerner Agency.
Lerner's other book, Food and Loathing: A Life Measured Out in Calories.