Endgame books have theoretical positions and sometimes exercises taken from actual games (although if you need positions from actual games you can even use your own).
With either type, set it up with a 10, 20, or 30 minute clock and practice it with someone. I've done this a few times in "dead" drawn endgames with a stronger player... and I've lost! Which is always a very good lesson for me.
Unfortunately computers aren't very good at endgames. But if the endgame is tactical or lends itself to calculation then computers can be a good partner. This is a small percentage of endgames though.
Everyone knows Carlsen is the best endgame player. He can produce wins from equal or almost equal position.
For the intermediate and advance players, how can we train our endgame skills? Endgame books are available, but how can we practice what we read? Suggestions are very much welcome.