Electron Transfer: ET classification

My research is going to reach some of the ET reaction, so I need to understanding the following questions:

1) what is ET?

2) how many kind of ET exist?

3) what is the common theory?

4) what are the recent developments?

Here, I just summarize those information from wikipedia, IUPAC, about the ET definition and classification:

0) definition [from Wiki]: 

    Electron transfer (ET) is the process by which an electron moves from an atom or a chemical species (e.g. a molecule) to another atom or chemical species.

1) Inner-sphere electron transfer

    The two redox centers are covalently linked during the ET process.

        If the covalent linkage is transitory, it is intermolecular electron transfer

        If the covalent linkage is permanent, it is intramolecular electron transfer

     Another definition is:  the interaction between the donor and acceptor centres in the transition state is significant (>20 kJ/mol).

2) Outer-sphere electron transfer

    The participating redox centers are NOT linked via any bridge during the ET event.

        If two redox centers are SAME, it is called self-exchange reactions

        If two redox centers are DIFFERENT, it is called cross reactions

    A reaction in which the electron transfer takes place with no or very weak ( 4-16 kJ/mol ) electronic interaction between the reactants in the transition state.

    There are five steps in outer-sphere electron transfer: 

        1. reactants diffuse together out of their solvent shells => precursor complex (requires work =wr)

        2. changing bond lengths, reorganize solvent => activated complex

        3. Electron transfer

        4. Relaxation of bond lengths, solvent molecules => successor complex

        5. Diffusion of products (requires work=wp)