I have been unable to find good, current general overview articles thusfar, but here are a few articles of interest.

Physiology and pharmacology of epidural analgesia. by Bromage PR. in Anesthesiology. 1967 May-Jun;28(3):592-622.
This is useful as an overview and historical perspective, but the reader must be careful to consider that much is now considered outdated.

Factors affecting the distribution of neural blockade by local anesthetics in epidural anesthesia and a comparison of lumbar versus thoracic epidural anesthesia by Visser WA, Lee RA, Gielen MJ. in Anesth Analg. 2008 Aug;107(2):708-21.

Sympathectomy-mediated vasodilatation: a randomized concentration ranging study of epidural bupivacaine.Ginosar Y, Weiniger CF, Kurz V, Babchenko A, Nitzan M, Davidson E. in Can J Anaesth. 2009 Mar;56(3):213-21. Epub 2009 Jan 28.