review of the Kondo effect by Barbara Jones. [It appeared in the Handbook of Magnetism and Magnetic Materials]. Much of the emphasis is on the two-impurity Kondo model which exhibits competition between the RKKY interaction I of the two impurity spins and the Kondo interaction J of each spin with the conduction electrons.
Of particular interest is the non-Fermi liquid fixed point (quantum critical point) which occurs for I=-2.2TK.
In 2006 Sam Young Cho and I published a paper in Phys. Rev. A which quantified the entanglement between the two impurity spins. A striking thing is that this entanglement vanishes at the quantum critical point. The quantum state of this model also provides a solid state realisation of Werner states [mixed states which have the intriguing property of being entangled but not violating Bell inequalities].