@howyoudoin: I'm on the same ballpark here, around 31 to 33 lines (kepubs, fullscreen mode, nearly null margins). I've always been fond of slab-serifs, so either Caecilia or Bookerly work well for me.
If I feel like full serifs, I either go for the classic, Adobe Garamond Pro; or the Crimson family, or the good old Gentium Book Basic.
But that's a highly subjective matter, so, YMMV

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