I read ‘I contain multitudes’. It’s not a nutrition book but it does has information about probiotics and probiotics. Might be worth checking out. It’s a bit discouraging of probiotics, but seems to indicate some prebiotics can be useful.

I’ve never been medicated for anxiety but I do have sleep issues that result in nighttime rumination (or vice versa, who knows). When sleep issues are untreated I’m generally much more stressed and anxious. So I treated sleep, and it means I’m also much less anxious as my mind isn’t locked into an anxious loop for hours an evening. I found an over the counter treatment that works for me (doxylamine). You shouldn’t take it chronically but my GP and sleep doctor approve it in my case and it’s probably got less side effects that a lot of mood stabilisers or sedatives.