This book is currently one the the best in print that thoroughly explores the evolution of Aktion T-4 into the Final Solution. I cannot recommend it highly enough.
As a Disability Historian and a Holocaust Scholar, my area of knowledge is Aktion T-4. T-4 was the Nazi programme of "euthanasia" against the disabled and mentally ill. From the beginning, Hitler wanted to empty hospitals and convalescent homes of so-called "useless eaters" and "life unworthy of life" to make room for the wounded of the coming war. By taking Negative Eugenics to its logical and fatal conclusion, Hitler and the Nazi Party took the vile step of exterminating those it deemed "inferior." At first, Aktion T-4 was limited in scope to so-called Aryan children but quickly expanded to included teenagers and adults with treatable disabilities and mental illnesses. With remarkable speed, the disabled and mentally ill of Germany, Austria, and France were rounded-up, sent from their home hospitals/institutions to centralized Killing Centers and gassed. Some scholars believe T-4 stopped "officially" because there was genuine and unexpected protest against the killings; but others believe it stopped because a target number of deaths had been reached. Either way, the Nazis were nothing if not efficient.
*Chelmno and the Holocaust: The History of Hitler's First Death Camp* makes it clear that not only did T-4 morph into the Final Solution, at times the two programmes overlapped. The staff of T-4 was trained in Killing Centers, and then transferred and relocated to help create and then streamline the assembly line system of extermination used in the Death Camps. It's no coincidence that both T-4 Killing Centers and the Death Camps and later concentration camps used gas chambers to murder. The T-4 Staff took what they'd learned murdering the disabled and mentally ill applied it to Nazi Racial Policy. Chelmno is the place became T-4 *became* the Final Solution. This has never been discussed and documented so completely in any English language work.
I bought *Chelmno and the Holocaust* for its documentation of the use of T-4 in Poland. I will reread it yearly as I do *Death and Deliverance* by Burleigh and Schmidt's biography of Karl Brandt. It's THAT important. We, as a society and as human beings, cannot forget that hatred of Difference, the Other and those we marginalize leads to death.