The wiring regulations are the authoritative source for the actual requirements.
If you are have a piece of information that references BS7671 and you are not sure what it means then looking up the actual wording in BS7671 can ensure that you are not misinterpreting as some authors take a very wide view of the wording!
The on site guide has basic information for simple installations, it also provides some general simplifications so that less calculation or thought is required to install basic circuits.
It can also provide some clarification (but also some simplification) of BS7671.
The guidance notes expand on the bare information provided in BS7671 and give more practical approaches to apply, these are just guidance from the IET, but they can prove to be very useful to interpret the sometimes unclear wording of BS7671 and also give some figures and calculations to explain more clearly.
Guidance note 3 is taken as the definitive reference for inspection and testing.
If you have a question about a specific item that is barely mentioned in BS7671 then the appropriate guidance note may provide a lot more details.
General reference books often provide a much clearer explanation of various requirements, however it is important to remember that these are the authors interpretation and it may not be agreed with by everyone. If there is some point that you do not understand then these books can often provide an obvious way of explaining an obtuse point in BS7671.
In all reference material, including the IET publications, watch out for errors, there are many mistakes made in many publications so do a sanity check before just plunging ahead.
It does help to know which part(s) of the wiring regulations applies to what you are researching so that you can then choose the appropriate reference sources.
For comprehension and understanding general reference books are easiest, for specific wording of the regulations: BSS7671, for expansions on BS7671: the guidance notes. For basic domestic circuits and easy answers: the OSG.
If you have specific queries then clarification can be obtained from electrical forums!
