Professional book appraisers earn their reputation and their credibility through years of experience. You don't just wake up one day and decide you want to appraise books for a living. Most appraisers spend years working their way up through the world of antiques and collectibles. They might start off as a shelver in a used and rare bookstore or a "picker" for an antique shop -- someone who drives around to estate sales and other antique stores looking for rare finds. Over the years, they gain enough expertise to start collections of their own and eventually open their own bookshops.
With enough experience, and close adherence to established standards of practice, the seemingly subjective art of book appraisal edges closer to a science. The most reliable appraisers are accredited by professional organizations like the American Society of Appraisers. Accredited members must take a test and vow to work in compliance with the "Uniform Standards of Professional Appraisal Practice," an annual publication put out by the Appraisal Foundation. In the U.S., there's no such thing as a "license" to conduct appraisal work on personal property, only real estate appraisals [source: Aimone].
It might seem strange that book appraisers would have to adhere to such strict standards, but that's because not all book valuations are done for the benefit of amateur collectors. In many cases, the Internal Revenue Service (IRS) relies on accredited appraisers to determine an estate's value. Lawyers call in appraisers during divorce settlements and insurance companies rely on appraisers when determining the level of coverage for personal property [source: AbeBooks.com].
Book appraisal doesn't require any specialized formal education. People enter the field from diverse educational backgrounds. A few colleges have degree programs or specialized coursework in the "valuation sciences," but most appraisers are educated on the job in the "theory, history and law of art valuation" [source: Aimone].
If you're in the market for a professional book appraiser, a great place to start is with a professional appraisal association, of which there are plenty.