put them in topical order. However, either one is kind of a complex business to get into, and there are other variations as well. Check out the last "page" or so of https://www.xml.com/pub/a/2004/03/17/dijalog.html
I have well over a hundred shelf-feet of books and they're sadly largely unorganized, following frequent relocations from state to state. Many of them are ordered by size (by force of necessity) and when I read them, not topic. I tend to keep truly professional career books segregated from other topics -- most of the former stay in my office, but not all, particularly if older works. I librarian I once worked for told me she keeps even her childrens' books sorted by DD; I've never felt the effort was worth the payoff. But I do need to put movable labels -- possibly on bookends -- to mark which topics are kept where, when that applies.
***ETA*** Since you're having your own shelves built, remember that equally spaced shelves may be the worst way to go ! I have all the biggest, heaviest books on the lower shelves, small paperbacks on the highest -- just in case the bookcase is ever tipped out of balance, it's less likely to fall. Try sorting your books by size -- you will have lots of certain standard sizes -- before choosing how tall the space between shelves should be. You will probably want one fixed shelf, a little below center, for stability, and the rest movable. Again, big boys on the lower shelves to lower the center of gravity. I even had two-tiered shelves, with the lower shelves much deeper than the upper ones, for large folio-type books etc.
P.S. I once read of an artist who sorted his books by the colors of their spines ! Visually impressive, but hard to search unless you have a strong visual memory.
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