The library is simple, you sign up for a library card (your name, email and phone number) and then you use their online catalog to browse for sketchbooks by interest, profession, location, color, mood, etc. The "librarian" (read: tight jeans, henley shirt, facial hair) will then bring you two sketchbooks at a time. Most of the fun is not knowing who's sketchbook you are going to get, and each sketchbook is inspiring in its own unique way.
My favorite sketchbook was by local Illustrator Greg Kletsel (see below). Anyone can participate by buying a sketchbook that cost $25--I assume part of the cost goes to maintaining the library--or $60 if you want them to digitize it, and submit it to the The Sketchbook Project, which to-date has close to 28,000 artists' sketchbooks.
I ended up buying myself a sketchbook to fill and a Pattern Box of postcards by 10 contemporary female pattern designers curated by the Textile Arts Center in New York. I've been super interested in pattern making and am currently taking Joshua Davis' Skillshare class on Generative Art, so hopefully I'll have some work to fill into my sketchbook!