When I was first getting into WW2 history, this was the first serious book on the subject that I read. It's quite long and detailed, but it provides a comprehensive look into all fronts and most important battles and events.
I suggest this as a starting point, as after reading you'll have a pretty decent understanding of the entire war and can focus on a specific front and/or battles that piqued your interest.
https://www.amazon.com/dp/0316023744/ref=cm_sw_r_cp_awdb_imm_MY74SHN3FJAHQ90SRXTX
No better place to start than Anthony Beevor's <strong>The Second World War</strong> ;)