http://www.signmuseum.net/library/topbooks.asp

A lot of the earlier books have carry-overs from the nineteenth century; unfortunately, a lot of these books are somewhat rare and, therefore, somewhat pricey...

Many towns and cities have municipal photo archives showing 19th century street scenes with merchant signage.
Some also have civic heritage organizations with similar resources.

This sort of thing:
http://pictopia.com/perl/get_image?provider_id=1072&size=550x550_mb&ptp_...

You might be interested in this collection of 19th-c. lettering I put together from the photos at shorpy.com.

This lovely book is about type from that era which was obviously influenced by hand-lettering.

The Handy Book of Artistic Printing

Also, similar to both Nicks' suggestions, check the local history sections in public libraries and bookstores. Most books of 19th Century photographs will have city scenes with signage.

The Hand Lettering Forum might be a good source of people restoring these signs or still using the techniques.