I started with the second floor, because I have very few books on the second floor, thus, an easy win. Walk around, find books that belong to me, see if they are in my catalog then add them if they are not. Notice that this does nothing to deal with the I-no-longer-own-it issue, but that's sort of a tough nut to crack so de-prioritize it.
Once the second floor was done, the first floor is the next logical one to tackle. I did a couple shelves off of one case, and realized I had a book in Dutch and a book in Frisian that were going to be tough to catalog, because I couldn't just use someone else's entry. I still haven't find a catalog entry anywhere for one of them (in Dutch but about Fryslan: het fryslan boek), but -after- I put together a catalog entry for De betoverhoed, I figured out that this:
CIP-gegevens Koninklijke Bibliotheek - Den Haag
meant that it ought to be appearing in one of the databases over on LibraryThing. And when I reduced the search to "betoverhoed", sure enough, it turned up (along with the Dutch edition from a few years later, not to be confused).
I still have several shelves to go downstairs (a couple of cookbooks, one of reference books and some other odds and ends), but I'll probably get it done this weekend, leaving the third floor (where almost all of the library is living) for some unknown future date.