Proper storage of your books can mean the difference between a valuable collection of fine copies and a disappointing batch of "reading copy" books, marked by mildew, foxing and broken spines.

• Books should be stored at a moderate temperature (60-70 degrees Fahrenheit) and at 50% humidity.
• Whenever possible, keep books on a sturdy bookshelf. 
• Shelve books upright. 
• Never shelve books too tightly. One risks damaging the book when replacing it or removing it from the shelf.
• Store books toward the front of bookshelves. This promotes proper air circulation and prevents musty odors from forming. 
• Avoid storing books near a heater, or in direct sunlight, as books may be damaged by extreme temperatures and harsh lighting.
• Stick a few whole cloves in the corners of bookshelves to prevent mildew. 
• Dust your books at least once a year by removing each volume from the shelf and, while the volume is tightly closed, brushing away dust with a soft, wide paint brush. 
• For maximum protection, consider purchasing a glass enclosed bookcase. 
• Avoid placing bookshelves such that they will be exposed to direct sunlight. Sunlight fades and yellows pages. 
• If your books have dust jackets and they are (a) very valuable, or (b) used often, consider using dust jacket protectors. These clear acetate sleeves are relatively inexpensive and do the job. 
• Dry is Better Than Wet; Hot is Better Than Cold... ...but both temperature and humidity extremes have risks. The ideal temperature is about 68 degree Fahrenheit with about 50% relative humidity (rh). To create a more book friendly rh in your library, avoid wide temperature fluctuations, as they are damaging to books (yet another reason to avoid the garage and/or attic) and consider investing in a humidifier or dehumidifier (depending of course on whether you want to increase or decrease rh) and a humidity gauge. Hot + Wet = Mold If you store your books in high relative humidity (70%+) and high temperatures, you are almost certain to encounter mold. If you catch the mold growth early, you may be able to dust it off and, assuming proper storage, prevent permanent damage. More advanced stages of mold growth cause irreperable damage, most commonly tiny brown spots called "foxing." There's no reliable way to remove foxing, so your only recourse is to store your books in a healthy climate (68 deg., 50% rh) to prevent further damage. v Another moisture-related problem is the dank smell of mildew in books. This occurs when the storage climate is too wet and too cold. To cure this, immediately remove the book to a drier environment, or crank up the dehumidifier. Then, allow air to circulate around the book. A portable fan set on low should work nicely. If all else fails, consider giving your book a little sun. This is usually effective for removing the unpleasant odor, but be forewarned, prolonged exposure to sunlight can damage paper, so proceed with extreme caution.

Infestations. Rescuing books from pests can be difficult. Many an insect enjoys lunching on all manner of paper products. If you discover an infestation, take action immediately! In order to provide useful information to the professional/s you will want to answer these questions first: Is the insect already dead or alive and how many insects are there? How many books are affected and with what kind of damage? Have you seen insects like these elsewhere in your home? Where have the books been stored and are they damp or moldy? How valuable and old are the books? Next, seal the affected book(s) in a plastic bag and make haste to your neighborhood entomologist. If you'd prefer to try to solve the problem yourself, here are a few ideas: lower the relative humidity with a dehumidifier, lower the temperature, limit access by sealing windows, doors, and adding a filter to each vent. Also, be sure to keep the storage area free of food or rubbish. 

We Are Our Own Worst Enemies... 

Probably the most common cause of book damage is simple carelessness on the part of the owner. To prevent damage: 

• Don't eat or drink while you read (or at least eat or drink neatly!). 
• Keep books stored on a bookshelf when not in use, this prevents them from being mistaken for thick and rather inviting drink coasters. 
• Don't lay your books face-down or, God forbid, dog ear the pages in order to keep your place. Use a bookmark!
• Avoid loaning your books out. You may never see them again, and if you do they may be in poor condition. 
• Consider purchasing inexpensive paperback copies of popular titles and loaning out those instead.

Source: http://rareandsigned.blogspot.com/2009/11/how-to-store-books-properly.html