Closer inspection reveals that a mirror is required to decipher this message, unless you're adept at reading backwards.
And then Waldschmidt signed her name in a legible left to write "Love to you both, Love Jean," which became illegible in the mirror image above, along with the rest of the left-to-right writing.
Leonardo and lefties. I learned from a bit of research that the practice, or art, of this kind of writing can be traced at least as far back as the 1500s to Leonardo da Vinci. Also, there is a name for it--mirror writing. Further, mirror writing may have evolved from left-handed writers (da Vinci was a southpaw), who had an easier time writing across the page right to left without smearing the ink as the writing hand dragged across the page in the process.
As for the mystery in the story... Someone is trying to stop a couple of teenage boys from helping with the demolition of an old Western pioneer hotel in Nevada. As they investigate who and why, they get caught up in solving an 85-year-old mystery in the old place.