What was slavery like during the world at war?

Slavery pretty much only existed in places like some of the Saharan emirates, and a few sub-Saharan African independent kingdoms. There would have been 'de facto' slavery (such as lifelong servants who would not be called slaves, but who basically had no options) in the very fringes of India and South-East Asia, but as a formal institution, it did not exist in most of the world during the time period you mention.

That said, you COULD argue that the USSR used Gulag prisoners as slave labor, and that Nazi Germany used its various concentration camp inmates as slave labor. Read Anne Applebaum's 'Gulag' for more information -- it's probably one of the best in-print sources that would break down slave labor in the USSR during the early to mid 20th century.

Well slavery had been abolished by the U.S, France and British Empire by then, so you wont find much.

However, as the answer below me said, you could argue that captured soldiers were used to do some minor labour.