Many of the caveats associated with the care of furniture can also be applied to the care of old books. Although tolerant of a fairly wide range of humidity and temperature, wide variations must be avoided: as well as the simple expansion and contraction which can break joints of books, the different materials in a binding will move at different rates, often leading to serious problems of warping.
The most delicate part of the construction of a book is the joint or hinge, which not only takes the load of the opened cover, but also is the thinnest and least reinforced part of the leather. The joints in particular suffer from drying out, the first symptoms of which are small areas of friable and usually white damage. To minimize this and other problems of a dry atmosphere, a regular but not obsessive application of a mild leather dressing, which typically contain lanolin and beeswax. Always try it out on an inconspicuous area of the book, apply very lightly at first, never use on sheep or reversed calf, both of which have a suede-like finish.
Problems of excessive humidity are usually more evident, but beware of damp conditions that can cause mold and mildew.
The least understood enemy of books is light. Not only does over-bright natural light fade books, but it also accelerates the drying process, which can be minimized by ultra-violet resistant films applied to windows, or by keeping curtains closed on bright days. When shelving books avoid leaving them at an angle, which can twist the bindings more-or-less permanently.
When it comes to handling books, always support the front cover when you open it, and never leave it dangling supported by its own weight. Never pull a book off a shelf by putting a finger on the head cap at the top of the spine.
If the covers come off, or pages become loose, do not attempt a repair yourself without proper training. Nearly all sorts of tape and many types of glue are inimical to books, and it is far better to preserve a book in pieces for restoration later than to improperly repair it.