I've bumped into this exact confusion more than a few times while chasing a favorite series, so I’ll be blunt: I can’t confidently name the books that follow 'Heartsong' without knowing which 'Heartsong' you mean. There are multiple books with that title across romance, fantasy, and inspirational lines, and some are standalone, some are the first in a duet or trilogy, and others are part of a publisher's themed line where titles share a name but not continuity.
If you want an exact list, give me one small extra clue — the author, the edition year, or even the cover color — and I’ll pull the sequence for you. Meanwhile, here’s how I track this stuff: check the author’s website (they usually post reading order), look up the book on 'Goodreads' and click the series link, or search the ISBN on WorldCat or Library of Congress to see related titles. If it’s a romance from a line imprint, the publisher’s page often shows the next title in the series. Tell me the author and I’ll sort the precise follow-ups for you.