An Introduction to Quantum Entanglement: a Geometric Approach
  We present a concise introduction to quantum entanglement. Concentrating on bipartite systems we review the separability criteria and measures of entanglement. We focus our attention on geometry of the sets of separable and maximally entangled states. We treat in detail the two-qubit system and emphasise in what respect this case is a special one. 
Karol Zyczkowski and Ingemar Bengtsson