Structured FM and DITA

[VLM TechSubs]

Hi everyone,
 
Thinking here about moving from unstructured FM to structured, and I realize
that I don't know what DITA even is, how it is related to structured FM,
etc. I would be grateful for pointers in the direction of good, introductory
reading in these topics, particularly DITA. Does not have to be
Frame-specific, I'm interested in understanding a bit about the whole
concept and mechanism.
 
Thank you kindly,
Elchanan