Honestly, I would recommend not books on atheism per se, but rather ones exploring non-theistic philosophy. Schools of thought amongst atheists and agnostics are just as diverse, if not more so than amongst theists. My world view is made up of many things, humanitarianism, empiricism and so forth. Ideas that build a foundation.
There is also the more emotional side, finding beauty and purpose in life. Personally I find myself coming back to the works of Carl Sagan, particularly “The pale blue dot”. Capturing, at least for me, a lot of essence.
Here is a short excerpt that he read. Just, wow.
Honestly I think the easiest thing would be to not allow images or embedding at all in PMs and perhaps display a warning message when clicking links “you are leaving [instance name]…”
Analyzing potentially lots of text and images in an effort to “guarantee” safety of users is likely a sisyphusian endeavour that is bound to fail - and furthermore also has privacy issues (namely that “private” messages aren’t private at all)