As a specialist in this material, I agree that for really secure
identifications, seek professional help.  On the other hand, I also like
to encourage fellow archaeologists to check out their plant remains on a
general level.  A basic familiarity with the frequently encountered plant
types in one's period/region is necessary and useful in crafting research
design and interpretation.

So, two basic references are:

Martin, A.C. and W.D. Barkley (1961).  Seed Identification Manual,
University of California Press.   - The "bible" for many seed types in
North America.  Reprinted very recently, although the photos are much more
clear in the original publication.

For those larger "nuts and pits", I would also recommend:

Young, James A. and Cheryl G. Young (1992).  Seeds of Woody Plants in
North America (Revised Edition), Dioscorides Press.  - This is an update
of the USFS Agriculture Handbook 450 (1974) called "Seeds of Woody Plants
in the United States", which is just as useful in most cases.

Both cover native and introduced species.

Hopefully available at a library near you.

Good luck!
Steve Archer
University of California - Berkeley