I see this type of topic come up a lot after being in the OHC hobby and lurking around these forums for over a decade. In that time I have stored my books both horizontally for several years when I didn’t have shelves, and vertically once I got them set up. There have been negatives to both. Granted I am not a professional librarian or trained curator so this is my personal anecdotal opinion.
When I stored my books horizontally I took extreme care to line them up so all edges were squared up and aligned, I also alternated the spines so that they were not all on top of each other. I stocked them 4-5 books high and tried to keep the thicker heavier books on the bottom to minimize any potential compressional stress on the binding. Despite the care I took and frequent checks and shuffling due to reading, I found many books started to “slide”. What I mean by this is that when viewed from the top a books front cover would start to protrude further out than the back cover. I don’t know if maybe the surface was not level or if there were some other unknown factor, but I am quite anal about my books and as I said I aligned them precisely also they were on top of an extra coffee table and desk I had, so the surfaces were stout and flat. I can only imagine what could happen on a shelf that may have some sag in it.
Once I got my shelves set up I put everything vertical which introduced it’s own issues. I will no longer store any of these books for a significant length of time without making “book shoes” for them. See my posts starting with #40 in this thread (
https://community.cbr.com/showthread...ected-editions) for the reason why. In addition to the page sag issue I have also noticed that the older heavier books have had the cover boards flatten on the bottom. It has been very noticeable on the older Omni’s that had the faux leather covers where the texture has been flattened and the edge has gained an almost shiny finish and the cardboard backing has been compressed to some degree making it so the case wrap can be bubbled up from it as it is no longer tight. By making the “shoes” you not only support the page block relieving the stress from the upper binding and keeping the page sag from distorting the pages, but you also distribute the weight across a larger surface and that should take the stress of the thin cardboard cover edges.
For a long time I would see people here and elsewhere talk about the potential damage long term storage of these books vertically might have on their construction and thought it was overblown. I have since come around, but I have also had negative effects personally observed from storing them flat as well. I find that the negative effects of vertical storage can be more easily mitigated than the ones I experienced from horizontal storage. Everyone is going to be different in what constitutes, or what level of degradation they are willing to accept in their collection. For me I am, as stated, very particular about the condition of my collection due to what I have spent on some whales, as well as what some books I bought new that have become whales, are valued at. Take my opinion/experience for what you may, but for how often this comes up I thought I would put out my experience from having been at this a while now and seeing the effects over a period of time from both storage styles.