I personally like the information that can be found in the MRG and bill bulfer books. However i have a massive caveat with these books.
They're not official source material so if you use them and it goes pete tong, when you're hauled in front of the magistrates you're up a creek with the proverbial. I've seen them but i've never used them in anger.
I'd only use them if I had a lot of spare time and all official material was exhausted. So QRH , tech manual and MEL consulted and still questions remaining. I think perhaps an example would be the LOT wheels up landing. (I know there are no bill bulfer or MRG guides for the 767). They had a problem, went around and had plenty of time to diagnose, unfortunately the problem wasn't found but perhaps a manual such as this would have provided an answer
I do personally think they are an excellent source of additional information though.