I rarely write about books, perhaps because there are few coming out that seem to fit the GM format. Still, two relatively recent publications should be of interest to modern guitarists.
Into the Maelstrom: Music, Improvisation and the Dream of Freedom, Before 1970 by David Toop [Bloomsbury Publishing] If you haven’t read Toop’s Ocean of Sound: Aether Talk, Ambient Sound and Imaginary Worlds, stop reading this and buy it immediately. For that matter if you care about modern music at all make sure you get around to all of his books. Into the Maelstrom is his long awaited first installment about the philosophy and practice of improvisation (both musical and otherwise). His erudite discussion leans towards England and Europe, but then so did much of the free improv scene. His personal relationship with many of the players makes the reading that much more interesting.
Visionary Guitars by Andrea Aguzzi [self-published] skews towards modern classical players like Magnus Anderrson and Seth Josel, but his interviews with Noël Akchoté, Joe Morris, and Amanda Monaco branch off into other genres. Like many self-published books, there doesn’t seem to be a budget for serious editing, or in this case translating, so figuring out what is being said can take some leaps of intuition. That said, Aguzzi is to be commended for intelligent questioning of interesting players.