There is a short chapter in R. Dupuy’s St. Vith Lion in the Way on artillery spotting planes titled Bumble Bees in the Fog. Unfortunately, the L2 and L4 light aircraft used for spotting are only mentioned briefly and just a third of the chapter talks about this interesting topic I’m guessing much of the original content was edited out as there is no explanation for the Bumble Bees title (probably received that nickname due to the sound of the engine).
I’ll summarize: the forward airfields were in danger of being overrun and the pilots needed to take their aircraft to safety while simultaneously flying reconnaissance and artillery spotting missions in the rare moments flying conditions allowed. Extreme bravery by the pilots flying in dense fog and somehow finding a landing site and an account of a recon mission flown along a road at tree top level, which spotted an approaching German column.
A comprehensive post about WW2 artillery spotting using Piper L-4 aircraft fills in where the Bumble Bee chapter stops with a set of links to rival the Bulge index. Bruce Gale fought in the Pacific but his experiences are relevant to his comrades flying in Europe:
“Bruce Gale often flew “figure 8” patterns while spotting. If artillery fire he was directing hit an enemy fuel dump, there would be a fire ball that would come up and he would have to fly in such a way that it would not engulf the plane, and although enemy soldiers tended to keep their heads down when the L-Birds were about, some fired up at the airplanes. On one sortie a bullet came through the floor of the L-4, traveled upward between Gale’s legs, passed through the bill of his “lucky” hat, and then exited through the top of the airplane. The incident was both shocking and thought provoking: Bruce Gale realized that one could get killed in his line of work.
Perhaps the aforementioned incident contributed to Bruce’s decision to arm his L-4. He had bazookas installed on each of his wing struts.”
Many pictures of the aircraft in this post, accompanied by technical specifications and operational history.