I don't remember what I was searching for but I came across Engineering Math 101 by WagMob. No I don't teach engineering at all but the free version has two lovely chapters on complex numbers. The material is in nice small chunks and make a great reference so students do no have to go through the book to find everything.

The first chapter covers complex numbers, imaginary numbers, Argand diagram, complex conjugates, adding and subtracting complex numbers, multipyling complex numbers, dividing complex numbers and the complex plane. Each has an explanation with either a diagram, examples or both. If you click on a topic, the app takes you down the page to the proper part.

The second chapter covers the polar form of complex numbers, Euler's Formula, De Moivre's Theorem and Powers of Complex Numbers.

Chapter 1 and 2 are in the learn portion of the app. There is also a review part of the app which has 7 flash cards to review 5 basic terms and 2 formulas.

The full app is $1.99. Unfortunately, this appears to be a reference app because it does not have a way for students to practice problems but on the other hand, I can send students to this app when they ask "Where in the book is this stuff?"