Syzmanski is a must, for sure. His book on Socialist Poland, and "Is the Red Flag Flying?" are respectively incredible books; the first gives a really good analysis of the Solidarnosc "movement" that most of the trot groups still defend, and the latter is the key riposte to the anti-Marxist ramblings about "state capitalism".
As for other ML historians, there's a bunch. Helen Yaffe is a must - I keep mentioning her book whenever there's a Cuba thread. Sam Marcy, whilst not a historian in the strictest sense, wrote a fantastic book about the counterrevolution in Poland.
Whilst not necessarily ML's themselves, there are some key authors and books on 20th century socialism which don't get mentioned when they really should. Key is anything by Wilfred Burchett, who wrote classic stuff on Vietnam, Korea and many other places in a fantastic style. Similarly, Anna Louise Strong did a similar thing for the USSR, China and so on, as did Han Suyin for China and Tibet. Ellen Brun and Jacques Hersh wrote the best book on the DPRK, and I am unaware if they wrote other stuff, sadly. Theodore MacDonald wrote incredible stuff on the education system and health system in Cuba, but sadly passed away this year. There are plenty others, but I'm away from my books right now so I can't think of much else off the top of my head!
All comrades should endeavour to look through the back catalogue of Monthly Review Press (and if you have archive access on their site, their old issues are great too), especially from the 1960s-70s, as this was when they released incredible stuff. Similarly, archives of Workers World, Fight Racism! Fight Imperialism!, Liberation etc all have great articles on stuff as primers to more in depth things.