This is much more difficult to do than any of the other general guides on the site because there are so many ways that the circuit may perform certain tasks so there isn't a rule of thumb as such.
There are a few obvious or simple things to try like different gain transistors and increasing input and output caps to increase the bass frequencies (or even lower them to decrease the bass frequencies), although simply changing the input and output caps may not necessarily do what you want if you are for instance already getting as much bass out of a circuit as a previous filter will allow.
Personally if I wanted to fine tune a circuit to my own tastes I would look for any high or low pass filters which allow me to tune the frequency response exactly to my liking, and maybe change a resistor value (or gain pot values maybe) here and there to get more or less gain as I like it.
Have a read through the Layout guide in the top menu, in that I described what was going in in the Eternity and it may give a few tips what to look for, particularly with opamp circuits.
The man himself gave his permission to FSB users to share them but didn't want them made public domain, ie. he didn't want people selling them but accepted that if people were going to pass them around he'd rather it be people who would get something out of them. Here's the link to a thread with Brian's original message and a couple of download links to his ebooks:
Looks like you're right, to be honest I've never been able to find a physical copy of this book that isn't going for crazy money but I haven't really tried all that hard. I'm guessing someone on FSB or DIYStompboxes will have a copy they may be willing to sell, maybe ask around there too.