I have the corgi manuals, Viper book and Bpec A5 folder.
The Viper goes into most detail but 500 plus pages of A4 is overkill IMo for using onsite.
The Corgi books have lots of extra info than the Bpec and are usual corgi book readability...
The BPEC is very well written and structured and covers all the main areas, but it depends how much information you want access to.
My summary!!//
Lots of Info (and some spurious stuff to make it harder to find) = Corgi book
Less info but most of its pertinent to a lot of jobs = Bpec
Huge amounts of info but too much for onsite apart from the fault finding one IMO - viper
hth