I've been rummaging around the internet; there seem to be two more books specifically about Wittmann ( that's if you exclude 'Tiger Ace'...)
There's one by James Lucas called 'Michael Wittmann' ( I think ) and then one of those massive J J Federowicz books by Patrick Agte called 'Michael Wittmann & the Tiger Commanders of the Leibstandarte' (ISBN 0-921991-30-4) - this looks like a fifty-quid book.
I haven't read either of these so cannot recommend - maybe someone else can comment ??
Also, there seems to be a Wittmann 'cottage industry' on the 'net. Some of them are rubbish but quite interesting is 'Wittmann's Attacke' on
www.home.t-online.de
Happy hunting !