I am a night owl and yesterday night I read an e-book from Luke Wroblewski about the Polar app which I enjoyed very much: It’s very interesting, well written, explained in detail and completed with cute, heartwarming illustrations.

Short summary why this PDF is worth reading if usability concerns you (and even if you’re not in app developing):

Frau Doudou

Frau Doudou is a web developer living in Munich. She loves working on creative solutions for sneaky web-problems – challenge accepted!
Feel free to ask questions and contact her on Twitter.