There is a great deal of Norwegian and Swedish folklore involving the lonely mountain occupation of the pre-industrial/pre-modern charcoal burners. In particular, legends describe his encounter with the skogsrå (Sweden) or huldra (Norway). You may want to look at these for insight into the occupation. These legends have the appearance of being quite old. See the work of Reidar Christiansen and John Lindow.

In addition, a monograph on the 1873 Charcoal Burners War in Eureka Nevada may be of use: Charcoal and Blood - due to be released in May 2016.

I have never found a book on this (wood-related stuff in the middle ages being a weird side-interest of mine.)

But try Roland Bechmann, Trees and Man: The Forest in the Middle Ages (Paragon House, 1990). The relationship of charcoal production and iron smithies means you might find some references in books on iron and arms/armour production, so Alan R. Williams, The Knight and the Blast Furnace: A History of the Metallurgy of Armour in the Middle Ages & the Early Modern Period (Brill, 2003) should have a bibliography that can help you find some research. There is a decent brief overview in John Steane, The Archaeology of Medieval England and Wales pp22-23. As Stean says himself, the evidence is mostly archaeological and not documentary, and not much research is funded on this.

I don't actually know of any comprehensive studies. But if you're going into the primary sources, note that the Latin ligna is firewood while materia is building-wood. So when Bobolenus says that St. Germanus of Moutier-Grandval "coepit ligna caedere, qualiter victitare deberent" (began to fell firewood, so that they might eat),[1] he means that the abbot busied himself with charcoal-burning to fund his endeavors. In general, a word search in the PL, AS, or CDST for ligna would be very helpful.

[1] MGH SSRM (ed. Bruno Krusch) v. 5, p. 36.