They should be comprehensive and full of details, without being dry and boring.
It would be great if they also contained plenty of personal accounts, reports, anecdotes etc from people (especially soldiers) who were involved in the fighting rather than just describing battle after battle without including the perspective of people who were actually there.
Alexander Werth was a Russian-speaking journalist who covered the Russian side of the war; his books are illuminating.
Guderian's Panzer Leader is a classic as well.
-Ivans war, by Catherine merridale -Bloodlands, by timothy synder
Chris Bellamy's Absolute War is a solid introduction to the Eastern Front that's well written and suitable for someone new to the topic.