Here is a super "clean" eBook on the 7 Drivers of Employee Engagement.  Short and to the point.  They say those 7 drivers of engagement are:

1. Purpose Alignment
2. Progress Inertia
3. Task Autonomy
4. Task-to-skill Alignment
5. Rewards & Recognition
6. Friends at work
7. Common Enemy

Here's the full eBook, enjoy!