Montefiore is a historian, in that he has a PhD from Cambridge. He is also someone who did extensive research for both of his Stalin books (Court of the Red Tsar and Young Stalin). However he is very much in the vein of a popular historian, and as such if you read his works take that into consideration. I do believe he has citations for both books, though if I'm remembering correctly they are not physically included but instead kept online (at the publisher's encouraging I would imagine). That said, he did spend considerable time actually doing research in Moscow and elsewhere for the books, though fellow Soviet/Russian historians have questioned that, unofficially (he makes a remark in one of the books about visiting a state library or something in Russia and walking through the shelves; that isn't how it works there: the librarians, who are nearly all older, scowling women, find the books for you after you fill out a request form).

With that all said, his books (I keep using the plural, because I think they are worth reading together; Court of the Red Tsar came out first, but Young Stalin is obviously the first chronologically) are worth reading. They hold up well, and while there is definitely a focus on the dramatic and an effort to make things more exciting rather than strictly analytical, they are not bad per se. Indeed for Young Stalin Montefiore is well-deserving of credit for both finding a lot of obscure, unpublished manuscripts in Georgia, and interviewing some of the few people alive (at the time) who knew Stalin while he was young. This is something that more "academic" historians openly acknowledge, and his books are cited by them. I also really found Young Stalin to be very engaging, and the way Montefiore wrote it came across like an adventure story or something (and reading Stephen Kotkin and Ronald Suny's later biographies only confirmed that the stories are largely true).

So if you are looking for a less academic-style biography of Stalin while he was in power, it's definitely worth reading. Just remember that it is written for a wider audience, and if you are looking for a more in-depth academic look at Stalin, I already mentioned Kotkin, who's second volume (of a planned trilogy) covers the period from 1929 to 1941 (right before the start of the war), and is very much an academic work (with literally hundreds of pages of microscopic notes and bibliography).