If you're interested in Stoicism, one of the benefits is that you can read (translations of) the actual Roman Stoics. I think the main thing is to find an author the meshes with you. A lot of people love Seneca, but I found him too long-winded and self-important (a bit like Polonious in Hamlet; where the advice may be solid by the delivery distracts from it).

My favourite by far is Epictetus. This site[0] has many translations of The Enchiridion side-by-side. I'd read a few of the options, find the one you like and just read that (personally I like Carter, and sometimes Long). It's short - 53 sections which are mostly a paragraph or two. You can knock it over in a few hours. I don't know if this is "optimal", but the approach I took was to read it all through fairly quickly, then re-read one section a day (or so) trying to really understand it.

Marcus Aurelius' Meditations is also quite readable if you find a translation you like. It's interesting to read the diary of an emperor as he tried to apply Stoicism to his life, but it didn't teach me anything that I didn't get from The Enchiridion.

If you want to ease in with a modern take, I think William Irvine's A Guide to the Good Life is decent, though he makes the mistake of adding his own more complex, and unnecessary concepts on top.

0: https://enchiridion.tasuki.org/

Not the commenter, but you can read Epictetus directly, very easy. Otherwise How to Be a Stoic by Massimo Pigliucci. Don't read any of that garbage from Ryan Holiday.