Critical mass: how nuclear weapons work
blog.nuclearsecrecy.com
blog.nuclearsecrecy.com
IAEA has quite a lot of reports on criticality accidents, just search their library[]. It's almost always sloppy experimenters not following their rules to get their work done faster and "easier". So, even if I don't get to work with fissible(?) material, it's a good lesson to learn for everyone working with dangerous machinery.
Most of their reading material is very good, and it's all free downloads!