If you want to understand leftism, look to Greta Thunberg
It is not easy to understand what the left -- as opposed to liberals -- stands for. If you ask a Christian what to read to learn the basics of Christianity, you will be told the Bible. If you ask a (religious) Jew, you will be told the Hebrew Bible and the Talmud. If you ask a Mormon, you will be told the Bible and the Book of Mormon. Ask a Muslim and you will be told the Quran.
But if you ask a leftist what one or two books you should read to understand leftism, every leftist will give you a different answer -- or need some time to think it over. Few, if any, will suggest Marx's "Das Kapital" because almost no leftists have read it and because you will either not finish the book or reject it as incoherent.
So, then, how is one to understand what leftism stands for?
The truth is it is almost impossible. What leftist in history would have ever imagined that to be a leftist, one would have to believe that men give birth or men have periods, or that it is fair to women to have to compete in sports with biological males who identify as females?
There are two primary reasons it is so difficult, if not impossible, to define leftism. One is that it ultimately stands for chaos:
-- Open borders.
-- "Nonbinary" genders.
-- Nonsensical and scatological "art."
-- "Music" without tonality, melody or harmony.
-- Drag Queen Story Hour for 5-year-olds.