What Lecture Notes Should Everyone Read?

There has been several questions with the same scheme as this one: What papers should everyone read What books should everyone read What are the recent TCS books whose drafts are available online what videos should everybody watch I was reluctant to post yet another one, but Jeff Erickson's lecture notes on algorithms changed my mind. I thought: Oh my! All these years and I haven't seen these excellent notes! So, I thought there might be other great lecture notes, which are really worth reading. So, for each computer science subfield ( data structures, algorithms, theory of computation, computational complexity, cryptography, etc.), recommend the superb lecture notes of your choice, and say why you think it excels. One simple rule to keep it tidy: One answer per each subfield. (This will be a community wiki, so you can edit existing answers, and add your recommendation.)