This book is deceptively modest in size, but manages a remarkably detailed overview of the geography and tree species of cold timberlines (though 'dry' timberlines in arid regions are discussed comparatively).
The book is divided into two parts, (1) Timberline Ecology and (2) Exploring Timberline. The first outlines the environmental factors which conspire to form timberlines - temperature, topography, wind, snow, soils and natural disturbances, to name a few. Part 1 also describes the evolved strategies of trees to survive at the edge of their physical limits, how timberline histories and trends are measured, and the human uses of the timberline environment. Part 2 is much longer and goes through specific timberline areas (e.g. the Klamath Mountains and the Adirondacks), focusing on the mountain geography, characteristic tree species and other features distinguishing a particular timberline.
The book is not for everyone: it is written in a textbook style, which is great if you've got an academic interest in forest or mountain ecology, or simply after meticulously-researched profiles of different timberlines. The amount of (fascinating) information, however, makes the book dense and is not one to read casually. Additionally, the book has a heavy bias towards North America (though the last chapter in Part 2 does briefly review timberlines in Northern Europe, Asia, the tropics and temperate Oceania/South America). While this doesn't dent the book's discussion of general timberline ecology, it could be disappointing to readers outside of NA.
Lastly, but certainly not least, the book is almost worth it simply for the illustrations. Although Hammerly's sketches are all black and white, they are very beautiful, accurate and on nearly every page.