Question about Sven Hassel's books

Is each book written by Sven telling a different non linked story or are they together forming one big sequencial war experience. Since there are some contradictions in the books which totally confuse me.

For instance in the 'Legion of the Damned' Porta was killed near the end but in the 'wheels of terror' porta reappeared as a living breathing person. And judged by the narrative 'Legion of the Damned' should happened earlier than the 'wheel of Terror' because there is a chapter in 'Legion of the Damned' which described Sven's first encounter with porta, pluto and the rest of the crew.

 

I did once sit down and chart out the chronology of his books (I have almost all of them) in terms of the battles fought etc - and it could actually be possible, even though he switches from front-to-front a bit (mostly in Russia, but not always the same sector, and also in The Med, in Italy and at D-Day).

IIRC one of the books was more of an 'overview' and discussed his entry into the Whermacht, his prison time, and them compressed all his Penal Battalion service into about half a book.