The Mountains of California (John Muir)
This is a truly unique book, which covers the Sierra Nevada mountain range that Muir knew and loved so well. Part geology and geography lesson, part travelogue, part guidebook, this book discusses the Sierras with a breadth that would require quite a few modern books from different genres. The writing is excellent and really sounds different - it almost seems like another language (it was first published in 1894). You can clearly see why Muir was such an influential figure. Great stories too - especially about his glacier climbing with no modern equipment.