A Skull at Springtime is one of Castle's bestselling novels written in the years before he met Beckett at the start of the series. It is one of his 'standalone' books, not part of the Derrick Storm series. Although referenced in the show, it has never been made available in the real world.
(From richardcastle.net) Looking for a way to pay for her college after her father’s death, Rachel Lyons is spending the summer planting trees in the clearcut forests of remote Washington. It’s a lucrative but lonely job and Rachel soon finds the monotony draining. That is, until she stumbles a half-buried skeleton deep in the woods – a discovery that leads her to uncover an entire field of corpses. When Rachel’s attempts to contact authorities are thwarted, it quickly becomes clear that she isn’t alone out here. As she struggles to escape back to civilization, Rachel must struggle to stay alive or risk becoming yet another one of the skeletons beneath the dirt.
References in Episodes
Flowers For Your Grave (episode 1.1) – When Ryan passes a stack of books to Esposito, Kyle's apartment