It's a 500 page book plus thousands of lines of source code, so your answers are in there ... somewhere. But the whole product is painfully disorganized.
The source code does not provide an adequate solution to producing Excel add-ins, it is just too brittle, hard to modify/maintain, and confusing. There are 141 functions that read "if Excel 2007 do this, if Excel 2003 or before, do that..." which would all have to be re-written every time a new version of Excel comes out. Instead of working around the Microsoft SDK files, he changed them directly, so again you'd have to adjust them to a new version of Excel. One of the big problems with the code was the author's choice to make his "cpp_xloper" class the center of the project, which contains a huge amount of code for moving data around through Excel's xloper/xloper12 data structures, including overriding the += operator so it adds numbers and concatenates strings. This was a big mistake in my opinion because it's ALL unnecessary. You only need ONE function that needs to know anything about xlopers -- the one that converts the data to be sent to/received from the Excel C API. That should be the only function that even cares if you're in Excel 2007 or before. All of the rest of the code should be done in normal C/C++ data structures. That right there saves you about 90% of the work!!
A large portion of the book is devoted to explaining this approach and documenting the source code, so to the extent the source code is bad, so is the book.
To be fair, after two weeks of going through a couple thousand lines of spaghetti code, I was able to write a nice C++ wrapper that does everything I want. The information is in there ... somewhere.