Herman Hesse summed up the appeal of fine books remarkably well in his Ideal Library: "Deep down, every true reader is also a bibliophile. For those who know how to welcome a book and love it with all their heart will also want to make it their own, reread it, own it and know that it is always close at hand".
This dynamic sector of incunabula (before 1500), antiquarian books (before 1851) and modern books offers some nice surprises, and it's not unusual to find a first edition or a fine binding (in parchment, morocco, chagrin, box calf, etc.) that fetches a good price.
Antiquarian manuscripts are also first-hand pieces of history, and can sometimes be a real surprise when the author and the theme are of particular interest.