Along with Book of Five Spheres, the definitive book of samurai philosophy.
Keep in mind that while Musashi (author of Book of Five Spheres) was basically the biggest badass of all time, Yamamoto Sensei (the author of Hagakure) was essentially a crotchety, retiring samurai, trying to revive the Golden Age when the samurai took their honor a little more seriously.
Good times, good times.
Web link of note: Hagakure(At http://split-kitty.com/misc/Hagakure/)