The BEST bookstore in the country (IMHO) is Strand Books, on the corner of Broadway and 12th Street. There's a huge art section on the second floor.

http://www.strandbooks.com/home/

I used to live a block away and spent many, many afternoon perusing the reviewers' hardcovers (50% off!). Now that my visits are much fewer and far between (it's one of the few downsides of living in California) I load up on all the irresistible art and type books my bank account can bear, and then arrange for Strand to send them to my home. They don't charge sales tax on the books if you do this, and the postage is pretty cheap. (I don't know if it's as good of a deal when they ship to Canada though.)

The Cooper Hewitt Design Museum has pretty good exhibitions. You might want to see if the show that's happening in November is of interest to you.

http://ndm.si.edu/

New York is fantastic; art and inspiration is all around. Have a wonderful visit!

The New York Public Library always has interesting exhibitions. Currently you can feast your eyes on a Gutenberg Bible and check out a show of "French Book Art/Livres d'Artistes: Artists and Poets in Dialogue":

http://www.nypl.org/research/calendar/exhib/hssl/hsslexhibdesc.cfm?id=411

Not sure what will be around in the Fall, but the building is well worth visiting regardless of what is on view.

The Morgan Library houses many objects of interest to designers including a handful of Gutenbergs and illuminated manuscripts. The space itself is, again, well worth the visit:

http://www.themorgan.org/

Coney Island is always worth a visit for any fan of lettering and signage. If you're in town early enough in the Fall, you may still find most of the stalls and rides in and around Astroland open on the weekends. Aside from all the older signage that's still up, an organization called the Dreamland Artist Club has been working on a project for the last few years to have artists and designers paint and build new signage for the businesses on the boardwalk. The results are super fun!