Probably the most important text on kratom is the college textbook which is dedicated solely to research about kratom. It is edited by Robert Raffa, who was pivotal in the elucidation of Tramadol. The book is about 400 pages.
PUBLISHED IN 2016
The most recent study I could find on mitragynine was published in the Journal of the American Chemical Society in May 2016.
Synthetic and Receptor Signaling Explorations of the Mitragyna Alkaloids: Mitragynine as an Atypical Molecular Framework for Opioid Receptor Modulators (2016) The full text PDF file is located here.
Kratom: A Collective Analysis by Dale Jerue (United Kratom Association)