What ultimately ended WWII? Was it the death of Hitler?

I assume you mean WW2 in Europe since the war against Japan raged for quite a few more months.

A good book on the end of the war in Europe is "The End" by Ian Kershaw. It looks at the days from the July Bomb Plot of 1944 until the war ended in May, 1945.

In regards to Hitler's death when he died Germany had almost no land left. Basically German territory was a bunch of cut off cities, etc. The war in Europe ended about a week after Hitler's death. Reichspraesident Doenitz (in his Last Will and Testament Hitler, who was Fuehrer und Reichskanzler, split that position into its original two and gave Reichspraesidnet to Doenitz, and the other, Reichskanzler, to Goebbels who committed suicide the following day but Doenitz had cut him out of government anyway) used that week to pull back as many troops as he could so they could surrender to the Western powers rather than the Soviets (civilians basically just had to fend for themselves).

Germany really fought until the bitter end. They had nothing really left by the end. It was really a total defeat. In his bunker in Berlin Hitler had at the end been ordering around divisions which on paper should have had 10k troops or more but had been reduced in strength to sometimes only hundreds of men. Even if Hitler had survived it's doubtful that Germany could have done anything more. It wasn't like most other wars where the losing power realises it's defeated and the government either surrenders, there's a military coup and they surrender, or the people rise up and they then surrender. Germany went down swinging.