I agree that the Field Guide to Produce is a lovely and very useful book. However, what's in season depends entirely on the region where the food is being grown. If you're talking about the local seasons (Chicago area), you might want to take a look at the website of Angelic Organics or some other local CSA (Community Supported Agriculture, or farm share) and see what they tell their customers about what vegetables to expect at what time of the year.
Here is the link to Angelic Organics. For some reason I can't reproduce the exact link to page with the vegetable calendar, but if you go to this home page and click around for "Vegetable Calendar" or "Vegetable Arrival Times" you can find a chart. You can also find a list of the featured vegetable for each week last year, which should give you some ideas.
http://angelic-organics.com/[url]
[/url]