Certainly the speaker meant to say “read between the lines” (to detect a hidden meaning), but apparently was also thinking of either books or beds. If the former, “don’t judge a book by it’s cover” (don’t prejudge something or someone from the outward appearance) comes to mind as books and covers are associated with reading. Also as suggested by my “ol’ pal” “cover to cover” (reading a book in its entirety). However, perhaps the reader was thinking of sleeping or other bedroom activities, conjuring up the slang idiom “between the sheets” (having sex). If so, I hope the speaker was wearing a Freudian slip.