The Ohio Department of Natural Resources publishes field guide pamphlets on a range of subjects. These paper booklets are about 7.5" x 5" in size, a little larger than a mobile phone.
These guides are unusual because they don't list species that aren't found or are very rare in Ohio.
Links to PDF versions are here and can be quickly loaded to a mobile phone or tablet on the trail, at a picnic bench, or at a campsite.