The Rules. All three of them.
Several years ago, when I had an appointment-only shop, a prospective buyer dropped by to take an in-person look at a book I had listed on the Internet. In its (extremely niche) subject-matter it was a moderately important book, and fairly scarce, at the time one of only three copies available online (there are none now). In terms of condition it was vastly superior to the others, but, mindful of the relatively small prospective audience for it, I had priced it competitively, in the mid three figures.
I brought it down from its hiding place on my dusty shelves and placed it on the counter facing the, er, um, "gentleman," who promptly flipped open the upper cover.
It emitted a faint but clearly audible sssss-nnn-aaaa-p as it fell onto the tabletop, whereupon the nice man flipped the book closed again.
And there it was — a distinct crack at the joint of the spine.
"Oh, well, it's broken," said the astute fellow. "How much is it now?"
(Needless to say, to this day that book holds pride of place on the corner of my desk, a reminder of the Evil Minions of Darkness who wander the earth breaking books for fun — and profit.)
Books are a miracle of functional design, made to be used, and used again, ad nauseam. But through the centuries, the materials that have gone into making them have changed dramatically, sometimes for the better, but more frequently for the worse.
If you've attended an antiquarian book fair, or if you collect pre-1800 books, it can't have escaped your notice that older books generally stand up better to the ravages of time than those of more recent vintage — for good reason.
At the dawn of printing, the making of books was very much a hand-craft, with every element, from the paper, the ink, the type, the binding, being made and assembled by hand. Building a book was a slow and painstaking process; merely printing a book could take weeks, even months. The materials used were of the highest quality, and built to last.
But as time went on, and literacy spread, the need for quicker, cheaper methods of bookmaking grew, and the codex became more and more of a commodity. Automation ensued, affecting the making of paper and type, methods of sewing the text block, and the materials used in bindings.
For the fascinating history of the development — or descent — of the craft of early bookmaking, see Nicholas Pickwoad's essay, "Onward and Downward: How Binders Coped with the Printing Press Before 1800," in the superb collection, A Millennium of the Book, edited by Robin Myers and Michael Harris.
Books, all books, not just rare ones, need to be handled with care. The materials from which they are made must be honored. Basic common sense — combined with a sense of the object's innate value (perhaps you don't mind breaking the spine of the pot-boiler romance novel you picked up for 25 cents at the local library sale as you ravage its pages!) — should govern the way you hold a book, open it, turn its pages, and store it.
Still, there are some basic rules which apply especially to rare books, which are perhaps less intuitive than simply "do no harm." Here are the ones which are (to my draconian mind, at least!) SIMPLY. NOT. NEGOTIABLE.
If you need to see into the gutter, or onto the opposite page, instead of lifting the cover further, try rotating the book by cradling the spine in your right hand while gently holding the cover open with your left. Particularly large books can be cradled in your lap, with the spine between your thighs and knees, leaving your hands free to fondle the pages.
And that's it. The Rules. All three of 'em. That plus a pair of clean hands and the mantra "honor the materials" will earn you a smile of gratitude from any dealer as you rifle through his (or her) stock.
But I'll close, just in case, with a final piece of advice, courtesy of Katharine Kyes Leab and Daniel J. Leab, as recounted in their essay, "Appraisal," collected in Jean Peters' indispensable Book Collecting: A Modern Guide (emphasis mine):
"Do not emulate the fellow who wrote ... asking about the value of a book, and enclosing the title page for identification purposes..."
Yeah. Definitely don't be that guy.