I myself do not really view “What is to be Done?” as a great beginner work for Marxists, since it mentions a lot of obscure philosophers or groups that a modern audience (with their cursory knowledge of Russian history being from the lips of liberals, or worse, conservatives) would hardly know the context of, and I am reading a version that has notes on these people!
That is not to say that it is not an influential or essential work of Lenin (I think it might be up there with “Imperialism, the Highest Stage of Capitalism” and “The State and Revolution” in terms of either factor), but one has to be willing to trudge through Russian names that you will likely never hear again.
“What is to be Done?” was not bad for me, but that it because I read Communism books and watch Communism videos beforehand. That said “Wage Labour and Capital” and most except the later parts of Imperialism, the Highest Stage of Capitalism was hard for me because I have a hard time understanding economic theory.
Those were definitely harder than I thought (they were both recommended as beginner books), but they were definitely useful. I really need to reread them someday.