Films included in John Springer's 1980 book Forgotten Films To Remember (1929-1959, with a 1960s-1970s appendix, containing the author's predictions as to which titles from that era are "in danger... of being forgotten.") Some of these are much less unremembered than they used to be, thanks to TCM, Criterion, preservation work  etc. And some are currently lost, though it's not always entirely clear whether Springer's selection/brief description of these is based on having managed to later see a now non-extant print, or, as seems more likely in most cases, from his memory of seeing those titles in original release. (On the other hand, several regarded as gone or nearly so 41 years ago, i.e. The Smiling Lieutenant and The Animal Kingdom, are now very much found, so never lose hope. Convention City's gotta be out there somewhere, right?)  

Note that the book only covers English language sound films. I grandfathered in Renè Clement's Le Repois because Springer includes it in its English language version, apparently all that was available in the US at the time of publication. 

In any case, still a good list for gleaning some undersung/interesting/'I've been meaning to get around to that one,' movies!

NB: I've missed getting one title in somewhere, I think, since the list is at 999 films, so it seems likely that Springer selected 1000 for inclusion, though he never gives a number. I'll go back and hunt it down at some  point soon.