There's a new book called A People's Guide to Capitalism by Hadas Thier that's pretty good. Michael Roberts, a Marxist economist who has a very nice blog, reviewed it here. Roberts also has his own book abt this, Marx 200: A Review of Marx’s Economics 200 years after his birth which is also a great intro book and is much shorter too.
Having also read Capital, Hadas Thier's book is an awesome summation of Capital if you don't have the time to delve fully into it. Explains all the key concepts in a super acessible way and gives relevant current day examples and applications. Also touches on anti imperalist and anticolonialist points as well. Only like 300ish pages as well if I remember correctly.
I've read the Roberts blog from time to time. Good stuff. Doesn't he consult and stuff for banks?
Mike does a good job laying down the fundamentals (which are necessary for any useful history of the Russian Revolution). For more detail, Breht and Allyson have a podcast called Red Menace (not to be confused with Red Scare) which digs into more detail on a bunch of works.
If you're an insufferable nerd like I am you may enjoy the manga adaptation of Das Kapital which is part straightforward explanations and part narrative story.
As an aside, feel free to post questions here from you're reading. There are a handful of very knowledgeable hexbears who will likely gonna jump at the chance to flex their knowledge muscles.
And the rest of us lurkers might get a little bit smarter to boot!
what concepts are you struggling to grasp fully? certain sources explain different concepts better than others
IMO going to the source is the best (yet most painful) option there: das Kapital.
Those are the things that people trying to do summaries often get wrong. e.g. use value vs. exchange value.
Might also check out Chapter by Chapter. It's about the most simplified you can get with Capital.
If you can read Spanish, Los conceptos elementales del materialismo histórico was helpful to me.
Read Engel's Socialism: Utopian and Scientific, its a good primer for people wanting to understand historical materialism without having to read Marx, Engels is much easier
In addition to all these other great suggestions, you can learn more about Marxism by playing the Class Struggle board game.