The book is Devils Guard by George Robert Elford and supposedly is the reminices of one Hans Josef Wagemueller a former Waffen SS Partisanjaeger who joins the lgion in French Indo China after the end of the Second World War and eventually commands a battalion of former SS types.
I own a copy and it is a good read. The first time I read it as a teenager I believed it was real, now I have my doubts. There is a lot of controvery around it.
http://en.wikipedia.org/wiki/Devil's_Guard
Fact or fiction it has been published quite a lot over the years.
Some confusion also surrounds the fact that there are at least two other novels out there with the same name including an Executioner series one by Don Pendelton. Elford also appears to have written a couple of “sequels.”
Looking at some of the prices they want I’m thinking of taking better care of my thirty year old copy.