This novel was published in the early-to-mid 1970s. Paperback. I am pretty sure the author was American and male. If I remember correctly, it was not marketed as 'science fiction' - it may have even been labeled 'romance'.
My problem: I began the book but didn't finish it! All I can remember is the opening scene - a young woman's mother has just died and she is going through her mother's effects. She finds a book - 'The Time-Life History of World War Two'. This is a puzzle, because there was no World War Two in this timeline. I believe the plot involves either time travel or cross-dimensional travel, I'm not sure which. I know it's not much to work with, but that's all I've got. Any help would be appreciated.