Oh Tiffany, there are some really wonderful, wonderful places. Here are my favorites:

Argosy has a little of everything and is always worth a visit. 116 E 59th (at Lexington, I think, or else Park).

J. N. Bartfield at 30 W 57th is excellent.

Bauman Rare Books at 535 Madison is good.

The Manhattan Rare Book Co. at 1050 2nd in the big antique market there sometimes has nice things and specimen books, mostly modern firsts though

Barbara Leibowitz Graphics has lots of illustrated books, posters and other ephemera and v. good prices. She only shows via appointment though but I am sure you can find her online. A lot of the better dealers in NYC will only show via appointment but the good ones will be very happy to talk to you and show you what they have.

Gotham Book Mart is still fun. 16 E 46th.

Leonard Fox has a lot