highly recommend the final chapter of this book - Dharma Transmission in Theory and Practice by William M. Bodiford. It explains clearly how the transmission was actually practiced and changed in Soto Zen. I actually found it fascinating.
William Bodiford also wrote a chapter about Manzan and how he reformed the institution of transmission in Soto Zen. If you do a google search using Bodiford's name and Manzan transmission - you will find his chapter on line at JSTOR... and i think you can read the essay for free on line. You can also purchase and download a pdf. I think both these chapters are essential reading if you are interested in Soto Zen history and the institution of transmission. Mostly what Zen students believe or think they know about the transmission is a mixture of idealism, fantasy, and grandiosity. Knowing more about the reality should be dis-illusioning in a good way.
Many people on this forum received transmission from Kennett or her various successors. I especially recommend these chapters to those folks.