Conversation:

I was under the impression that 'on the road' was one of a trilogy. Any ideas what the other two books were called?

The first one is 'Desolation Angels', which is the story of Kerouac's life just before the publication of 'On the Road' (as told through his fictional self, Jack Duluoz).
The second would be 'On The Road', and the third one is 'Big Sur', which continues the adventures of the (older but certainly not mellower) wandering beatnik Jack Duluoz.

...although, at the beginning of Big Sur, I'm sure there's an introduction by Kerouac in which he says that his work is all of a big series.

Is Go/The Beat Generation by John Clennon Holmes available anywhere?

After reading these three, you should try 'Off The Road' by Carolyn Cassady. It's her autobiography about her life with Neal and Jack, giving you a fresh view on the events in 'On The Road'.

Subsequent to reading all those four it would be worthwhile reading
"The Electric Kool-Aid Acid Test" by Tom Wolfe which describes Neal Cassidy & Ken Kesey leading 'The Merry Pranksters' over America.