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Hidden in the fourth Waverly Jones book, Silent All These Years, is a reference to a non-Waverly book by Skyla Dawn Cameron, which suggests these books might take place in the same universe.
All three copies have been claimed.
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In the chapter “A Case of You” when Waverly comes home from East Arbroath, Sebastian is reading a Doc Malarky medical thriller. Although the author isn’t mentioned, it’s by Norah Sloane from Dweller on the Threshold.