There's a lot of love here for the 42nd St. library, but I'm personally a fan of the performing arts library at Lincoln Center. Aeron chairs, tons of open space with plenty of outlets, and if it's warm out you can work in the Lincoln Center plaza for a change of pace.

If it's warm out and you don't need internet access, I'm also a big fan of working from the High Line. There are lots of shaded tables, and you can't beat the ambiance. If you need to duck away for power or WiFi, Chelsea Market has both nearby (although getting a place to sit at Chelsea Market can be tough if it's after 11am or so).