Lenin was pretty radicalized in his home. Literally everyone in entire his family were more or less active revolutionaries. Death of his brother Alexander seems to be major step to radicalisation of Lenin.
I would suggest to grab Collected Works and read early articles, there are some bits here and there.
What is better, is not his radicalisation, but his evolution later. From absolute nerd spending months and years on writing about statistic, to fledgeling politic and publicist, gaining experience in this through conflicts with mensheviks, coming into the big stage in 1905, then changing into the true leader of people and finaly successful head of state after revolution.
Maybe his letters?
No, there’s not much there. Most of them of that period did not exist or survived because most of meeting before his exile were face to face and those from prison were destroyed by recipients (usually his mother and sisters). There is a lot of letters from Siberia, but those are usually very mundane about technical issues with publishing things or daily things like going to bath, walk or hunt.
One things he seemed to spam everyone is “SEND MORE BOOKS”.
read about how his brother died
