I was just alerted to an updated resource today, _The Guide to Iowa's State Preserves_. I bought an old copy (didn't look used at all) from a book sale this winter. It was printed in 1992 and entitled _Iowa State Preserves Guide_. By using this page at the IDNR website, I was able to download a digital pdf copy of the guide to make my paper book current to 2001. You can still purchase a hardcopy guide from U of I Press http://uipress.uiowa.edu/books/pre-2002/herguito.htm (There have been five preserves added since then, and they will be added later.)
In order to have a geocache in these areas, citizens are encouraged to create an Earthcache. You can read more here: http://www.iowadnr.gov/parks/rules_regs/geocaching.html
If you're in my area, you can check out Strasser Woods near the state fairgrounds. My girls and I drove up to it on Sat. It's one of those places that you always see the sign for, but just drive on by. It definitely looks worth a hike.