I can't speak for the book, never having read it, but "be still my heart" is a fairly well known idiom. In contemporary English, the expression is usually used sarcastically to imply that a situation is not nearly as exciting as has been suggested. Edit: Sorry about the messed up link.
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HSSI don't see it grammatically. Is this an imperative?Yes. It's an imperative. The speaker is talking to himself / herself, saying "I must calm down. I must control myself. I am overexcited", but it takes the form of commanding his /her heart to beat more slowly.