  In this paper, we investigate the behaviour of statistical physics models on
a book with pages that are isomorphic to half-planes. We show that even for
models undergoing a continuous phase transition on $\mathbb Z^2$, the phase
transition becomes discontinuous as soon as the number of pages is sufficiently
large. In particular, we prove that the Ising model on a three pages book has a
discontinuous phase transition (if one allows oneself to consider large
coupling constants along the line on which pages are glued).
  Our work confirms predictions in theoretical physics which relied on
renormalization group, conformal field theory and numerics
([Car91,ITB91,SMP10]) some of which were motivated by the analysis of the Renyi
entropy of certain quantum spin systems.
