There are not many books on GaN and here is one. However, the book is nothing but a list of existing (old) work, with little understanding of those work or analysis of those work provided. Throughout the entire book the author uses very vague words like: "..... is modified/changed", rather than providing any definite information like "it is increased/decreased by two order of magnitude", "improved/degraded by a factor of 100 times", or in a more complicated situation, providing a graph stating there is a peak at a certain condition. One can easily pick up examples in any pages, such as in page 156 "A device annealing is performed at 500oC and leads to a substantial CHANGE of the barrier height and the leakage current"; P157 "Hydrogen is found to be dissociated on the Pt surface and forming atomic hydrogen, which MODIFIED the effective barrier height" etc, and no graph/table is provided. The impression after reading the book is something has been done by someone, but the reader will have no clue if the work make a positive or a negative conclusion or any conclusion at all. There are also paragraphs or sentences apparently missing (such as the last paragraph in page 185 is not connected with the first paragraph in page 186).
Overall, the book is far from finishing. It is essentially nothing but an old reference list and might save the reader a little time in finding references. But remember, the references are not quite complete even for the old publications. I would rather believe that the book is pushed out publications for some other reasons. Probably one should wait until the 2nd edition is published and more detailed information is added.