Regarding Ivan's files, which are certainly the core and gem of the collection: it is not easy to precisely count [ex. there are a few instances where it looks like we have a hole-punched pile of things which have "lost" their notebook], but my count is 302 "books".
The bulk of these have science supertitles ["Biology", "Geology", "Cultural Anthropology"] and contain whatever Ivan was interested in saving that he thought might apply to odd events/sightings. These number 63, 28, and 30 notebooks respectively. Within them are often materials which you or I would see as directly related to certain anomalous claims [ex. the stuff related to possible existence of dinosaurs currently is in a science notebook not a "monster" one.]
"Monsters" however DO show prominently. There are about twenty notebooks on miscellaneous "monsters", mostly water-dwellers. But the "ABSMs" dominate both the numbers and Ivan's interest, as books labelled so number 33. UFOs get 31 notebooks. Things associated with other sciences get 18. Miscellaneous anomalies [ex. Falls; Psi], get 31. There are 8 bibliographic notebooks, and 41 where Ivan collected certain journals, largely UFO newsletters. This numbers slightly over 300 notebooks or seventy-five-and-a-half-feet of linear shelf space. The anomalies research community can breathe a happy sigh of relief about that.
Now since you've been so nice to read all this "pragmatic" informational report, I'll toss in an example of a "neat thing" that one occasionally finds in the files. This is the picture of the "giant Congo snake" allegedly taken from the air by a bush pilot around 1960-2. Somehow a good "original" got into Charles Hapgood's and then Ivan's hands, and they were sending it to someone to try to make a competent estimate on how big the thing really was. The pilot had said "200 feet". Hapgood's letter to "Captain Burroughs" is included below for your interest. I'll get back on other SITUations as I can.