I manage several large Excel files that are updated multiple times each day. Rather than rely on macros or VBA, I'd like to programmatically update the data by extracting the relevant zipped sheets and streaming through them one at a time.
That's easy enough with XLSX files since their components are all XML. There are many resources for doing this with XLSX files. There are very few (if any?) for XLSB files because their components are all binary.
How does one convert binary files to human-readable formats, or parse them in a human-readable way?
Being a binary file, the XLSB file is not easy to parse. The file format is complex. You should use an Excel library that knows XLSB file format and EasyXLS is one of them. It also knows XLSX file format.
I don't know what is your favorite programming language, but this link about importing Excel file in C# is a good start for your needs. You can use a similar code for both XLSX and XLSB file formats.
If you prefer to keep your current code, you can use the library only to convert the XLSB file to XLSX file.