I missed this discussion earlier, but yeah… if there is one book that is directly tied to The Dark Tower it’s Salems’ Lot. I wouldn’t say it’s required reading before diving into TDT, but it’s the biggest tie-in by far. A lot of the other books (with the tower keyhole on them) have cameos or plot points that run alongside stuff, and the whole thing oozes suggestions to read deeper and find Easter eggs… but, to avoid spoiler territory, I’d just say that Salems’ Lot is the most vital part of the expanded universe narrative