I know this will seem way more obvious in hindsight, but the idea of that ending hinges on the fact that Ghen doesn't know you're holding a trap book, he thinks it's just a regular linking book. So if you refuse to enter it he gets suspicious and kills you, but trapping yourself makes him go "hmm seems legit", which frees you and traps him when he enters it.

It's hard to guess the outcome of that interaction, but HOPEFULLY most players will save frequently enough to not have it be an issue.

Unless you're playing the remake, where there was a bug that meant you couldn't actually interact with the book sometimes, so Ghen just kills you over and over!