Well, since Destroyermen and The Man In The High Castle were mentioned, how about John Birmingham's Axis of Time series?
It's a pretty great Isekai kind of dealie similar to Destroyermen, but instead of an obsolete USN steamship (and others) from 1941 getting transported to a Lost World type universe where lizardmen and lemurians are fighting each other in a war of extermination, it has an entire modern (near-future at the time the book was written- it's vision of the future got some things right, like Prince Harry joining the military, while others were way off, like Hillary Clinton being elected President in 2016) naval battle group en route to do a police action in an Indonesia taken over by Islamic State-types gets transported back to WW2, literally right in the middle of the Battle of Midway in 1943, and after doing a number on both sides due to the confusion it joins the Allies and proceeds to help win WW2 as well as try and uplift the downtimers to their technological and social standards. Things do not go according to plan. There are five books; I've pictured the first.
Also, if your looking for some things far less SF but still fitting into "the fictional books about 20th century conflicts" criteria, how Harold Coyle's Team Yankee, Robert Conroy's 1945 and Jon Kacer's Festung Europa?
Team Yankee is set during a mostly conventional WW3 and follows a group of US Army tankers fighting against a Warsaw Pact-USSR incursion into West Germany during the Eighties. It's really good, and about the only thing I find unrealistic about it is that both sides backed down after the nukes came out.
1945 takes place in a world where the coup attempt launched against Hirohito after he announced his plan to surrender succeeds, and the Emperor is held hostage by IJA hardliners who refuse to surrender even after a third atomic bomb is dropped, necessitating the launching of Operation Downfall, the proposed invasion of the Home Islands that was projected to be so bloody they only recently ran out of the Purple Hearts that were minted for it.
Festung Europa (Fortress Europe to those who don't know) is set in world where the Nazis don't commit forces to N. Africa to bail out Mussolini and end up winning the war against the Soviet Union and fighting the Western Allies to a standstill while Japan is simply starved out instead of nuked and eventually surrenders a year or so later. A "Warm War" between the Western Allies and the European Axis ensues which goes Hot eventually, and the Second World War ends in 1960 with Europe ravaged by WMDs and the final starving Nazi holdouts are crushed by Allied shock troopers after refusing to surrender. It is amazing.
