Some internal LucasArts documents have recently been put online by Aric Wilmunder.

Here's the official manual for SCUMM: http://wilmunder.com/Arics_World/Games_files/SCUMM%20Tutoria...

And the official tutorial: http://wilmunder.com/Arics_World/Games_files/SCUMM%20Tutoria...

There are several post-mortems about Maniac Mansion that go a bit into the creation of SCUMM: * https://www.youtube.com/watch?v=wNpjGvJwyL8 * http://www.gdcvault.com/play/1014732/Classic-Game-Postmortem...