He's up to 34 pages of useful circuits, in color, using commonly available parts. These mesh perfectly with the Maker movement - you don't always have to be an engineer to make circuitry for cool projects. To be honest, most engineers keep a notebook of basic, helpful circuits. Alberto is just kind enough to share his in such beautiful drawings. For clarity, select the PDF versions. Or go to his Flickr site for pictures.
He is most receptive to comments and revision suggestions on his blog.
While you are there, also pick up his wonderful Arduino pinout diagrams. (Use the menu to find all of the others).