Athanasios C. Antoulas, Approximation of Large-Scale Dynamical Systems
When I have started learning model reduction, I have found many excellent paper about MOR by Prof Antoulas
They helped me a lot to understand the theory behind MOR. The book has appeared in press a bit too late for myself, already at the end of my work at IMTEK, yet I have really enjoyed it. It gives very good introduction into the subject and I can highly recommend it. Well, it is mathematics and one should be ready to invest time in reading and understanding equations but it is after all impossible to understand MOR without this.
If you find the price of the book too high, it is still possible to find Selected Papers on the homepage of Prof Antoulas and read his papers.