Cruising Under Sail by Eric Hiscock is one of the most information dense books I own; and I own a LOT of reference books. Though some of the material may seem dated (not mention of modern fibers for line, contruction remarks heavily favor classic wood techniques, etc), this book is one that really should be on every sailor's shelf. I refer to it constantly.
Oddly enough, I rarely see it referenced on online forums and web sites.
In addition to the shear volume of practical information, one thing that makes this book extremely useful is that Hiscock wrote based on decades of cruising experience with tens of thousands of miles under his keel. His approach is therefore less "cruising style" and more "cruising substance." In contrast to a lot of contemporary cruising literature (Lin and Larry Pardey notably excepted), his style is not one of "spend money on this gear or that gadget." Though not a project book, this one is a great read for the DIY-er that lives somewhere inside practically all cruising sailors.