This was a chapter book I read in the mid 70s about kids that built and then ran a mobile disco. I think they called the disco 'Frankenstein' on account of it being made of bits from everywhere. Their tutor (who ran one already and showed them how to build such a thing) would give tips on how to handle the crowd.

I remember one tip:

If an Elvis fan comes up put an Elvis record on assuring the fan "Elvis is The King' because as his logic went, you are the entertainer and that is what the fan wants to hear.

Definitely not a real 'How to' book, more a tale aimed at kids. Aimed at my then age, then about 11-12 years. Anyone?

I recall getting this book out of the kids' section of my local library or my school library. A kids' adventure book, (from an era when kids had adventures) I'm guessing this book was published late 60s, early 70s

This book is definitely 70s. It's not 'Crazy Gary's Mobile Disco' from 2001.