Not sure how I should play this one...

[TheFaQQer]

It will obviously be worth 1/2 a day of my time to him...

Sounds like your best bet - explain it to him that you need to spend time removing information from there that is information in the notebook that is company confidential that you are not willing to share. Make sure you take your time copying and sanitizing it - I'd pad the 1/2 day out to a day and not bother charging for the book instead.

From both a principle and practicality point of view, it's important that you keep a copy of the information in the book - you never know when you might need it.

As an aside, I've just bought myself a digital notepad - write on paper as normal, and it records it as a page onto memory or SD card. Handwriting recognition extra, but all I want really is the page as a JPG that I can store somewhere at the end of the project - customer can have the paper copy if they want, and I'll have the digital one.