I can confirm the same observations as chrizn was telling: I get a big file with now more than 140 pages and starting AP I can observe how my mac is getting hot and disc space occupation is increasing relatively rapidly to full (occupying more than 100GB just by the program). As it is a photo book there are many photos linked (up to now about 4.8 GB) what would not justify the amount of disc space occupied (it multiplies the linked data by 20!) . Photos and work file are on the same drive (not my internal hard drive!). So I get the same messages as chrizn is showing and many times the program shuts down without notice (5 minutes later I get a collapse message). Even the program now closed the occupied hard disc space is not freed but I have to restart my mac to get it off. This seems to me a big bug, first of all that the Publisher is keeping all the data tracked despite only to be linked to the original files (this should definitely be less than the sum of all linked files!), and second not to free this space after closing the program.
As I couldn't work / progress with my big book I had to start a new file (and hopefully can combine the two parts afterwards into one). With this it works fast and without that kind of space usage (but still with few pages)
My specs: MacBookAir 13" 2018 with OS 10.14.6, 16GB RAM, 500GB SSD, APublisher 1.8.6