Sometimes old classic books are very short, like these two I recently read for free online:
St. Patrick, Confession (St. Patrick's autobiography) https://confessio.ie/etexts/confessio\_english#
Sun Tzu, The Art of War http://classics.mit.edu/Tzu/artwar.html
I was trying to think of a good romance I've read recently. There's a romantic plot (between Jason and Medea) in the second half of Apollonius's Jason and the Argonauts (aka Argonautica), although it spoils it a bit if you know the tragic ending of the relationship from Euripides's play Medea. But it's a pretty good adventure story about Jason getting the Golden Fleece with Medea's help. Here's the translation I read: https://www.amazon.com/Argonauts-Penguin-Classics-Apollonius-Rhodes/dp/0143106864