It’s a big month for celebrating milestone anniversaries for a number of well-known literary agencies. Writers House went first on Tuesday night to commemorate their 40th, at the Morgan Library, joined by over 500 people. (The event also served as a surprise 83rd birthday celebration for Writers House founder and chairman emeritus Al Zuckerman.) Over the course of September, additional agency events will honor Inkwell Management (10 years old), Curtis Brown (100 years old), and Levine Greenberg Rostan (25 years old).