I don't think the air dryer mentioned in capttb's post is a true dehumidifier, it's a small heater and warming the air will tend to keep more moisture in suspension.
A true dehumidifier will need to have accumulated water removed periodically or drained somehow.
We've messed around with the small compact 'dehumidifiers' and after frequent failures have moved to a typical room-sized model from Sears or whomever. It's a bit of a pain to carry out to the car for off season trips but it's done a stellar job of keeping the boat dry. We run it at 45-50% setting and books and paper stay crisp, no mold or mildew anymore even in our damp environment here in Vancouver. In shoulder season times there's enough waste heat to keep the edge off below.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"