Reddit reviews Jasenovac and the Holocaust in Yugoslavia

The Catholic church openly collaborated with the Ustaše regime and supported it, especially on the start of the war. There is only few of the members of the church who actively participated in the killings, Miroslav Filipovic being the most prominent one. This is a text in Croatian about him and some of the worst butchers from Ustase regieme, but you can use google translate: https://www.express.hr/life/fra-sotona-i-najgori-ustaski-koljaci-7443

The policy for the Serb population by the Ustaše regime was to kill one third, banish one third and convert one third to Catholicism.
There was a number of massacres in which the Serb population was going to baptism to Catholicsm hoping to be saved, but was instead killed. One of the worst is in Glina: https://en.wikipedia.org/wiki/Glina_massacres
"On the night of 30 July 1941, a massacre similar to the one in May again occurred in Glina.[18] That summer, the Ustaše had offered amnesty for all Serbs in the NDH who would convert from Eastern Orthodoxy to Roman Catholicism. Many Serbs responded positively, and one group turned up at a Serbian Orthodox church in Glina where a conversion ceremony was to take place.[27] The Serbs who had gathered, thinking they were to undergo a conversion ceremony, were greeted by six members of the Ustaše[27] under the direct command of Vjekoslav Luburić.[28] When all were inside, the doors to the church were sealed. The Serbs were then forced to lie on the ground as the six Ustaše struck them one by one on the head with spiked clubs. More Ustaše then appeared and the killings continued.[27] Victims were killed by having their throats cut or by having their heads smashed in with rifle butts.[18] Only one of the victims, Ljubo Jednak, survived after playing dead and later described what had happened."

There was of course some clergy that helped prevent the massacres and wrote letters in protest and in efforts to stop the massacres. Some smaller part even joined in the Partisan movement. You can find more in this article (again in Croatian, use google to translate): http://www.prometej.ba/clanak/povijest/katolicka-crkva-u-nezavisnoj-drzavi-hrvatskoj-2370

I hope i helped a bit :) Regarding the books here are some in English: https://www.amazon.com/Jasenovac-Holocaust-Yugoslavia-Barry-Lituchy/dp/0975343211
Here is a author from outside Balkans whose works you can check to avoid Balkans based bias:
http://www2.le.ac.uk/departments/history/people/akorb/publications

These 3 books I recommend: Jasenovac - Auschwitz of the Balkans by Gideon Greif, The Yugoslav Auschwitz and the Vatican by Vladimir Dedijer, Jasenovac and the Holocaust in Yugoslavia by Barry M. Lituchy and 44 Months in Jasenovac by Egon Berger.

Although I think that you could just Google "Jasenovac concentration camp" and "Sisak children's concentration camp"(the only concentration camp during WW2 dedicated specifically for children) and read some articles about that. I also think that even Wikipedia pages for these two are sufficient enough and you don't have to spend money on books.