Here is a very informative thread on this case opened by Isaac Koi last June at ATS. It includes a link to a free online pdf of Richard Haines's book on the case obtained by Koi and links to video clips, Valentich's interactions with ATC, and extracts from other sources concerning the case.
Valentich disappearance - rare book now online - "Melbourne Episode" by Dr Richard Haines (free), page 1
Valentich disappearance - rare book now online - "Melbourne Episode" by Dr Richard Haines (free), page 1