Hah, I actually haven't used it since I was last doing misc consulting stuff before living in Moscow in 2015-2018. I found my BestBooks folder easily enough, but had an ancient SheepShaver. Downloading an Apple Silicon-compatible build from January 2025 I replaced the app file, pointed it at a disk image, dragged the BestBooks folder in via the Unix shared folder ... and it launched right up!

Here's browsing some oooold data...

https://hoult.org/bestbooks.png

https://hoult.org/bestbooks_about.png

BestBooks is apparently an offshoot of MYOB 3, your find is MYOB 5.

You should upload it to macintoshrepository.org. It seems like it is not available anywhere else.