"My point is that if we cannot trust any single source, we cannot trust the majority either."

You can never trust any one source totally [except me, of course]. That is true.[ Which is why in any self-respecting book, there's usually a bibliography, some of which can be very long. You have to consult widely to produce a book that's going to have value.]

However it doesn't follow that you cannot trust any source in any way.

A part of a researcher's work is to know HOW to use sources of information. Thus non-professionals like some of us are at a disadvantage because we haven't leaarny how to evaluate sources, which is the crux of the matter.

However you can trust some sources pretty much - like documentary records on Public Record Offices. And anything which has been peer-read should be relatively free of error.