If you're trying to build the next big app, you need user engagement. This book lays down a model building engagement by having users constantly return to your app. In the beginning this is prompted, but eventually it'll become instinct. This is how viral loops are formed.
It lays out the "Hook Model", a basic framework of the 4 key stages of each loop:
1. Trigger: How does the loop initiate? In the beginning this may be through external triggers (such as an email, notification, icon badge, etc) but through successive loops the user eventually creates internal triggers where a particular thought or emotion will send them back to your product.
2. Action: Once the user is aware they need to use your product (through the trigger), what it the simplest action they can perform to get some kind of reward. For example a Facebook "Like".
3. Variable reward: How are they rewarded for this behavior? This could be social validation (e.g. "my friends approve!"), collection of material resources (e.g. add a photo to a collection) or personal gratification (e.g. inbox zero). The "variable" part is important - rewards should not always be predictable, encouraging users to repeat the cycle.
4. Investment: Finally, the user needs to put something back in to increase the chance of repeating the loop. This could be content (e.g. a book in your Kindle), user entered data (e.g. profile information or linked accounts), reputation (e.g. something to gain a 5 star seller review), or a learned skill (e.g. I'm now really good at this software program). The investment also sets up the trigger to for the next cycle of the loop.
This book is a really easy read. I wanted something that would get to the crux of the problem and set out a practical framework of how to apply it with examples, without being overly verbose on history and research. It delivered.