I use a Lignomat 'Mini Ligno'. About $75 on ebay these days. My unit uses a series of LEDs to signal from 6% to 20% in 1% increments, you can get ones with an LCD as well. Comes with two sets of pins, but I put the longer of the two on (about 7mm) and left it that way for all readings. It has a trigger switch between the pins that alows you to uniformly probe to the same depth and take an accurate reading while at that depth every time. Pretty simple! Bought it for doing wood flooring and general woodshop use, but works great on firewood too. Readings I usually get on bone dry maple are around the 12-16% range dependant on ambient humidity/time of year (up here its pretty dry in the winter and fairly humid all summer). I'll burn anything that tests under 20%.
Anyhow, check out the Lignomat line of lignometers. Worth the slightly higher cost, but they are a real tool used in real trades to make real money.
fs41