Leninism doesn't quite depart from Marxism really, Lenin expanded upon Marx's ideas and applied them to the evolving material conditions of Tsarist Russia. In a manner of speaking he departed with Marx in the way that Marx thought revolution would occur in developed industrialized nations like Germany and France and not rural agrarian societies like Tsarist Russia. Another thing to note is a lot of what Lenin wrote was scathing and sort of exaggerated polemics against others who he often worked alongside strategically, but argued with for the purpose of directing the course of action correctly. His ideas, the conditions and people he was responding to, evolved over time as well, so it's very important to understand those contexts going in, and this book does an excellent job of synthesizing and condensing some of that well:
https://www.goodreads.com/book/show/26268757-revolution-mani...
I think this does a good job too: https://www.goodreads.com/en/book/show/50284837
I would also recommend The People's Forum / The Socialist Program's recent class on Lenin: https://m.soundcloud.com/thesocialistprogram/sets/lenin-and-...