For decades, I knew Colin Fletcher's The Complete Walker was a classic and had a significant effect in publicizing and popularizing backpacking to the masses beyond the cadres within the Sierra Club, AMC, etc. but I hadn't read it. When I did recently (I read two different editions), I was impressed by how advanced his strategies were in each edition - he was ahead of the curve on many UL techniques, weighing and measuring everything before selecting gear, and field-testing the bajebbers out of everything. With the kind of miles he did, he had to find efficiencies where he could.
There was, for me, a nostalgic aspect of reading reviews of 20- and 40-year-old gear (packs, stoves, etc) that I grew up with and later upgraded too. And, even with hindsight, I'd agree with almost all of his recommendations of specific makes and models.
Of more general interest and a classic by anyone's definition is his "The Man who Walked through Time" - the story of his 6-month trip, the first ever documented, of the entire length of the Grand Canyon.