Is understanding tensors a prerequisite for understanding plastic deformation (metal forming) equations, plane stress, plane strain conditions, etc..? If so which book should I use to understand tensors?. Because on the internet people suggest that a good resource to learn tensors are the physics books that deal with relativity, etc. So I am not sure whether that perception will be suitable for studying the stress tensors.
In a mechanical engineering curriculum at university, you would probably take a series of 3 classes. First would be called "Strengths of Materials" (or maybe "mechanics of materials"). This would be a sophomore level class. No tensors, just linear algebra (matrices). The second class would be "theory of elasticity". This is going to involve tensors. Third class will be "Continuum mechanics". This will be very heavy on tensors. After that, there would probably be a specialized class on plasticity. Depending on your comfort level with the math and your background, pick a text book on one of these subjects.