M. E. Chaber is in reality Kendall Foster Crossen, who wrote SF and crime novels under several names, not to mention hundreds of TV scripts. As Chaber, he wrote about Milo March, and insurance investigator who sometimes got called back to active duty by the army, as is the case in The Splintered Man. The year is 1955, and March's assignment is to go into East Berlin to search for Hermann Gruss. Gruss is the head of the counterespionage police in West Germany, and he's supposedly defected to the Reds.
There was no wall in those days, and March doesn't have much of a problem getting into East Berlin. The trick is to get back out alive. It's not easy, and it's complicated by the fact that he's caught and given LSD. I don't know this for sure, but I'd be willing to bet that Chaber was one of the first writers to use that drug as a plot device. The cover's a bit misleading, however, as the dose is administered in a glass of water, not with a needle.
The plot is okay, and in fact it sounds a lot like this one, but with a different location. Chaber has a smooth style, and his Milo March novels are what I'd call medium-boiled. They had a revival in the '70s when Popular Library reprinted them with McGinnis covers that had March looking very much like James Coburn.