Wong has a good book on theory of off road vehicle dynamics tracked and pneumatic tires:
What type of suspension are you looking to design? IFS? Solid axle/leaf?
What jounce/rebound travel are you aiming for?
Dont limit your choice to off-road books because there is more literature available from the racing side which is fully applicable to off-road suspension systems.
Google basics of vehicle dynamics, which includes selecting your base dimensions (track, wheelbase), target ride, roll stiffness, etc...
has a heavy truck dynamics and light truck/vehicle dynamics seminars.
The Gillispe book already recommended is an awesome book. Milliken and Milliken make the next best books available to suspension engineers: Chassis Design (Olley's notes), and Race Car Vehicle Dynamics.
There are enough books out there to fill your life with enough reading material.
Let me know if I can help you some more.