No problem. I was also bugged in the same way. Luckily, I happen to have "Modeling the Bipolar Transistor" by Ian Getreu (Tektronix, at the time) written circa 1979-1980. It's an excellent book on the BJT and covers detailed procedures in the last half of it for deriving the various parameter values from specific instrumentation and measurement procedures he discusses. That last half is actually the more valuable part of it, as I've not found any other single source discussing very specific means that one can actually set up on their bench for measuring BJT parameters up through Gummel-Poon. But it also includes this Isat equation, too, in the very first few pages.
Jon