X11: How does “the” clipboard work?
is a technical walk through the modern X11 selection system, one that winds up discussing things
at the level of the X protocol and Xlib, with helpful code examples.
I learned some quite useful things in the process, for example how
to use xclip to query things
to find out what formats a selection is available in.
(Technical details about X selections are relevant to me because
I use a program that deals
with them and which I'd like to see do so more conveniently.)