2-key prefix and Invalid Prefix keys in sequence

[Tim Johnson]

Hello:
I'd like to map out a series of 3-key commands, with
control-c control-; as the prefix.
Initially I am attempting the following:
(global-set-key [(control \c\;) (b)] 'isearch-forward)
;; evaluation gives me the "Invalid Prefix Keys in sequence"
;; error message
1)How can I make this simple sequence work?
2)There is extensive info docs on keymapping, and I've seen
  it briefly, but can't find it again. :-( What's the command
  to get to it?
3)And the *bigger* picture is: I ultimately want to set up a
  global (or local) keymap with a 2-key prefix (such as C-c C-;).
  I've found numerous examples of how to *add* a key to an existing
  keymap, but no examples of how to create a new keymap and
  new prefix key.

All help and pointers to docs are greatly appreciated.
TIA
tim