Theoretical Introduction to Physics with Linear Colliders
  The major open questions in particle physics are summarized, as are the abilities of linear colliders of different energies to add to the knowledge obtainable from the LHC in various scenarios for physics beyond the Standard Model. A TeV linear collider would provide much additional insight into electroweak symmetry breaking, in particular, and a multi-TeV linear collider would add even more value in all the scenarios studied, for example in models with supersymmetry or extra dimensions. 
John Ellis