I did some searching online and here's what I found...
According to this site
"Dry, seasoned wood is best. The type of tree is not as important as
moisture content. Even the most dense wood must be allowed to dry for a
minimum of 12 months after being split and stacked. Popular firewood in
the Northwest are: maple, alder, Douglas fir and fruitwood, such as
apple and cherry. Another good fuel source is pressed, fire logs,
manufactured by pressurizing sawdust, as opposed to wax-based
decorative logs, which have limited heating value."
's cached version of this site
confirms the last one, saying that "The best is hardwood which has been
split, covered and air-dried for one year. Logs that are 18-20 inches
in length and 6-8 inches in diameter are best for burning."
Lastly, this site
points out that "When building your fire, remember that hardwoods burn
slowly and evenly. Softwood, such as pine, should not be used for
anything more than kindling when starting a fire, as it burns too hot
and too quickly. You should always keep a combination of dry and green
wood on hand. The dry wood for cranking up the fire and the green wood
for slowing it down some. "
(republished on 07/24/06)