If you already have a good working understanding of the BS7671 Regs then the On-Site Guide does exactly what it says on the cover. It's a great little quick reference for the day to day run of the mill stuff. However it is essential to make sure you are already up-to-date with the full BS7671 17th Edition to fully understand what this guide DOES NOT cover. For special locations, correction factors etc. you still need the full Regs, so don't get this book as a cheap alternative, but rather as a working companion, to the Regs.