I'm confused!
There may well be an unfinished book which has yet to be published but PLF's next book, after those mentioned in the question, was published 20 years ago!
http://www.amazon.co....qid=1308519269&sr=8-1
Chris, he planned a trilogy recounting his walk to Istanbul as a teenager. But they were exceedingly slow in coming - he did it before WW2 - and the third was still somewhere in the works when he died. However, his executors say there's enough to publish.