https://archive.org/details/historicalmateri035186mbp - gentle (but not oversimplified) introduction for those who are interested in this subject. And another book recommended by the author (Nikolai Bukharin) to study before a historical materialism: https://archive.org/details/abc-of-communism_202102
These are his early writings on theory, in which he takes a strictly Marxist-Leninist view. The rudiments of right-wing bias and so-called economism can be found in his other, much less popular work: https://archive.org/details/politicseconomic0000buch