In the latest installment of “Book Keeping,” a series of articles available exclusively to mediabistro.com’s AvantGuild subscribers that reveals the backstories behind recently published books, Marisa Meltzer and Kara Jesella spill the dirt on Sassy, or, more precisely, they explain how they came to write How Sassy Changed My Life, a history of the influential ’90s girl’s magazine. As Meltzer describes the process, “It’s one of those things where you can’t believe your job is to sit and drink tea in Green Point and discuss Kim Gordon and the 90s, and her incredible importance.” Hell, I’m practically an expert at all that, except the Green Point part; clearly my problem is that I’ve been giving it away…