An excellent and freely available 181-page "textbook" about modern cryptography. From the preface:

"This is a set of class notes that we have been developing jointly for some years. We use them for the graduate cryptography courses that we teach at our respective institutions. Each time one of us teaches the class, he takes the token and updates the notes a bit. [...] The viewpoint taken throughout these notes is to emphasize the theory of cryptography as it can be applied to practice."

Topics include: Block Ciphers; Pseudorandom Functions; Symmetric Encryption; Hash Functions; Message Authentication; Authenticated Encryption; Number Theory; Asymmetric Encryption; Digital Signatures; Key Distribution; Zero Knowledge Proofs; and more.