J.H. Slater's Book Collecting - A Guide for Amateurs (Swan Sonnenschein, London 1892) concludes with a still useful chapter - 'Books to Buy.' The author regrets that there is no device (vademecum) 'capable of being carried in the waistcoat pocket which will enable him to spot a rarity at a glance...' This was just over 100 years before smartphones which, to some extent, now fulfil this very purpose (and if the book has a barcode there are also applications that will emit a noise telling you to buy.) The reference to the need for a register of 'scarce but mean-looking' English books (now known as 'sleepers' and which every good book scout or 'runner' has in his or her head) concludes with a florid latin quotation concerning glory..
Slater starts by mentioning the pathologically acquisitive bookseller Naude and the rich bibliomane Heber...
But Naude had the wealth of Mazarin at his back, and free licence to purchase as and where he would at the Cardinal's expense, while Heber was rich beyond the dreams of avarice;the modern book hunter, whose means we will suppose are limited, must discard the yard measure and the scales, and rely on his judgment, taking care to get the utmost value for his money. He will have to make up his mind to buy or not to buy on the spur of the moment, for while he is consulting his books of reference at home, a golden opportunity may be missed. This is his capital difficulty, and one which it will take years of experience to surmount, for there is no vade mecum capable of being carried in the waistcoat pocket, which will enable him to spot a rarity at a glance ; nothing, in fact, which can compensate for a lack of practical knowledge.
I have often thought that a register of scarce but mean-looking English books, of such a convenient size as to be carried in the palm of the hand, might be of assistance to those who haunt the stalls, and delve among the rubbish usually to be found there ; some day, perhaps, it may be worth while to try the experiment, sed Gloria, quantalibet quid erit ; si gloria tantum est? What will be the value of ever so much glory, if it be glory and nothing else ?
In turning over the contents of an old book-stall,the major portion of the heap will be found to consist of volumes of sermons, and other theological treatises, recipe books, odd historical volumes, and poetical effusions, besides periodical literature of the Spectator and Tatler brand. Books of this class are, as a rule, merely rubbish ; but still there are a few exceptions. Sermons of John Knox and Dr. Sacheverell, or any of Mather's tracts, are invariably worth purchasing ; as also are first editions of sermons by Cardinals Manning or Newman. Early editions of Mrs. Glasse's cookery book, or any recipe books of the seventeenth century, may safely be speculated in ; so may early editions of poetical works, if written by authors whose reputation subsequently became established. Third, fourth, or later editions are seldom of much value, no matter who the author may be, and no matter of what character or description, provided they come under one or other of the heads enumerated above. In purchasing books of the class generally found on second-hand stalls, there are two preliminary questions to be asked : first, was the author of sufficient reputation to make his name well known ? and secondly, is the particular copy of his works offered for sale an early edition? If an affirmative answer can be given to each of these inquiries, it will be advisable to tender the small sum likely to be asked, and to run the risk.
Another point to be observed is that where a printer's device appears on the title-page, or indeed on any other part of an o/d book, it is more likely than not to have a value, and it ought never to be passed over without a careful scrutiny. Should the collector be fortunate enough to pick up a rare French book, his best policy will be to have it suitably bound in France by a first-rate binder. Though already valuable, its importance will be still further increased by this manoeuvre; for when the inevitable day of parting shall arrive, the French bibliophiles will be more inclined to welcome native talent than any English imitation of it...
Volumes containing separate tracts should always be examined, as it sometimes happens that rare pieces are found bound up with a mass of worthless matter. I once heard of original editions of two of Moliere's plays being found in this way ; and as these stand pretty much in the same position, so far as rarity and consequent value is concerned, to the early Shakespearean quartos, the importance of the " find " to the lucky discoverer can hardly be exaggerated. This is only another example of the rule which can never be too often repeated, since it can never be sufficiently understood. If the author is "big enough," and the edition is early enough, buy.
The probability is you may not realise the full importance of what you have got until you have had time to consult some book of reference ; it may indeed turn out that a wretched and dirty reprint has done duty for the original, or it may so be that the book is worthless on its merits. This is one of the risks of book collecting, and, it may be added, one of its charms. Hundreds of thousands of dead and forgotten books must be annually disposed of, for nominal sums, in London alone, and there is no telling how often these and others may have been turned over and flung aside by passers-by before they eventually find a market. Among all this profusion of rubbish, a certain percentage of valuable pieces must necessarily exist... Every year some of these princes in disguise are rescued from the wind and rain, and henceforth considered a fair exchange for gold instead of copper ; but alas! we cannot both eat our cake and have it too. "Finds," as they are called, are not so numerous as they once were, nor hucksters so ignorant as in the merry days of Dibdin and Button, to say nothing of such foreign Nimrods as Colbert, Cirolier, and the great Pixérécourt.
The same rules which guide the haunter of the stalls are suitable to those who purchase from the regular booksellers. There is so much to be learned, so many artificial rules and distinctions to be observed in everything relating to books, that mistakes are of frequent occurrence. Ignorant assistants have before now unwittingly thrown shabby little books, like Burns' Poems (Kilmarnock, 1786), into the sixpenny-box at the shop door ; others have been too lazy to sort the parcels as they have come in from the auctioneers, and have bundled the whole contents into the same repository. There are a hundred and one accidents in favour of the book, hunter, but he needs experience in order to take advantage of them, and this cannot be got without the expenditure of much time and money and the suffering of many disappointments...
The full text of the book can be found at the invaluable Internet Archive.