I get a little giddy mapping out this series, because the way the author spreads
lore across main volumes, gaidens, and one-shots makes for a small treasure hunt. If you want the smoothest ride, follow publication order first: read 'Undead Yokai Girlfriend' Volumes 1–12 in order (they collect Chapters 1–120), then the two-volume gaiden '
Moonlit Promises', then the light novel 'Letters from the Grave', followed by the side-story collection 'Afterlife Tea Time', and finally the epilogue one-shot 'Eternal Morning'. That path preserves reveals and character development exactly as readers experienced them.
If you're curious about internal chronology, slot the prequel one-shot 'Yokai Origins' before Volume 1, tuck 'New Year's Specter' (the holiday one-shot) after Volume 2 for a cute detour, read 'Afterlife Tea Time' between Volumes 4 and 5 for extra context about the supporting cast, and drop 'Moonlit Promises' between Volumes 8 and 9 if you want its backstory before late-series twists. I usually stick to publication order for the pacing, but sometimes I binge the prequel and then savor the main run — either way, the characters land beautifully for me.