If I may be allowed to say so, whilst some valid points about the history of book production have been raised in this thread, some wild generalisations have been made too; given the magnitude of the subject, I suppose that’s excusable.
“Many old books are now brown due to the acid in the paper while most of my new books are on acid free paper.”
It depends what you mean by old, Daivid. Up until the end of the hand-press era, a point which roughly coincided with the advent of machine-made paper (around the second quarter of the 19th century), the overwhelming majority of books were printed on handmade paper, which, unless it has been stored in a harsh environment or subject to rough usage, will generally have lasted extremely well. This is not to say that machine made paper cannot be made to a very durable standard, just that it often wasn’t and isn’t, and I’d wager that the “old” books you refer to that have browned almost all date from the mechanized era. Handmade paper can brown too, but it is rarely a result of acidity; usually it’s a consequence of the particular kind of plant fibres that formed the pulp, or the nature of the size that was used to “harden” the finished sheets. Vitally, the bonds between the fibres of handmade paper will almost always still be sound, even if the paper has browned, whereas the bonds between the fibres of machine-made paper that has browned as a result of acidification will often have weakened considerably.
Books today are frequently printed on acid-free paper, yes, but such paper is unlikely to be as durable as good handmade paper for two reasons: firstly because even if pH-neutrality is achieved (and sometimes your wonderful acid-free paper is actually strongly alkaline) the neutrality is achieved by the addition of buffering agents in the pulp rather than an absence of injurious chemicals in the first place; and secondly because the paper will, except in very rare cases, still be machine-made, and machine-made paper by its nature (the nature of the paper fibres and their alignment in the sheet) is inherently weaker than handmade.
One other crucial factor that is often overlooked by the buyer who has noted with satisfaction that the book he has just bought is printed on acid-free paper is what method has been used to join the pages together. I have lost count of the number of books I have seen boasting that they are printed on acid-free or pH-neutral paper but which are merely a block of single sheets held together at the spine edge with adhesive - a glorified notepad, in effect, destined to early disintegration.
“Hence the reason why we are being offered these very poor quality scans is because the printing of 100 years ago was often appallingly bad.”
I would disagree with that. It’s certainly true that printing from 100 years ago was sometimes appallingly bad, but more often poor reproduction in a reprinted book today is a result of failings in the technology employed in the reproduction or failings in the way that technology was applied.
“My advice is to avoid reprints and get the originals when possible. Often they are actually quite readily available.”
“Printing quality must have always varied, just like it does today.”
I’d endorse both of those statements.
“The best we can do now is certainly better than it was a hundred years ago.”
I’d have to disagree with that. The best we could do now could certainly equal what was being produced a hundred years ago, but firstly I don’t see it being done anywhere on any significant scale, and secondly there is no technology that can improve upon the legibility and near-permanence of the best letterpress-printed books from a hundred years ago, whether they were printed on machine-made or handmade paper.
This debate is not over, I’m sure, and I’ve only scratched the surface myself, but at least I’ve had a say; “too much of one”, some of you might be thinking.