I've found both the 3com site and the Intel developer's site to be useful.
It still takes digging, but I've been lucky.  The smaller companies are
a problem.  Many of their products are `clones' and they don't even bother
to write a manual.

Also, the PCMCIA, PCI, USB, and other consortia provide the baseline
description of much of this stuff.  The vendors often just say what
parts they really do.  ANSI for the ATA spec, for example.  Since
there are many vendors playing in the same space this makes sense.
Back when only DEC defined what a VAX was, each peripheral's documentation
would fully describe that peripheral.  Now they say `vesa video controller'
and just describe what makes them different.

In 1996 I had top secret copies of one of Intel's chipset I was using.
Today you can the equivilant document off the web.

  Brantley