how many nations contributed to the SS forces during WW2? Did they only serve on the Eastern Front? After the war, how were veterans treated? Were they jailed or executed? Were many forcibly conscripted or were there ideological reasons that pushed them to serve?
I wish we knew more about them. It seems mostly a dirty secret for many Euro nations to overlook this.
In what way is it overlooked?
I can't speak for every European country just taking Scandinavia, here in Sweden, Bosse Schön's documentary book Svenskarna som stred för Hitler ("the swedes who fought for Hitler") was a bestseller the 1990s, and also resulted in a documentary TV series that got good ratings (I remember watching it, and talking with classmates who'd watched it about it). An extended and revised book was republished as Hitlers svenska soldater in 2005. On the more academic side of things there's Lars Larsson's Hitlers vikingar: svenska SS-officerare (2015) as a recent one.
In Norway the photo-heavy "Norske frivillige i Waffen-SS" came out just last year. Another recent one is Sigurd Sørlie's Solkors eller hakekors nordmenn i Waffen-SS 1941-1945 (2015) which is supposedly the most well-researched book as of yet on the topic as far as Norway is concerned. On a more personal level Bjørn Westlie's 2008 biography Fars krig about his father fighting for Germany in the war won awards and was rated the sixth most important war book in Norway since 1945.
In Denmark, Christensen, Poulsen and Smith's Under hagekors og Dannebrog : danskere i Waffen SS is on its fourth edition (2015) since being first printed 20 years ago. Leif Davidsen, Karsten Lindhardt, Østfronten : danskere i krig (2005) is another recent addition. And of course, Denmark had Sven Hassel who's books about his supposed wartime experiences as a volunteer for Germany were bestsellers in Denmark in the 1950s and '60s and did well internationally as well. Whether Hassel actually served is highly questionable but irrelevant to the point that at almost no point was anyone pretending there hadn't been Danish volunteers for Germany.
This is not intended as a comprehensive list, just some the most recent major works on the subject; there are books going back decades. I would assume that other countries like the Netherlands, Belgium and so on have their own literature on the matter. As a European I find it offensive that you're claiming it's overlooked 'dirty secret'. It's neither. You're being glib.