First of all Thank you for your book "OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide". Without this book I wouldn't have cleared my OCP 8 exam.
Congratulations and Thank you for writing OCP Oracle Certified Professional Java SE 11 Programmer.
Recently I started looking in to 1Z0-817 objectives and realized that it covers many of Java 8 topics too. Couple of months ago I donated my OCP 8 book to local library as I already passed my exam.
Is the new book "OCP Oracle Certified Professional Java SE 11 Programmer I Study Guide" sufficient to prepare for 1Z0-817 exam?
There are some chapters in our first book that will help you on the upgrade exam. For example, 1Z0-817 exam contains the module objectives of both 1Z0-815 and 1Z0-816. So reading the modules chapter in our 1Z0-815 book will help with half of the module objectives for the 1Z0-817 exam. Same with our chapter that covered var in the first book.
That said, there's a lot of things that are not covered (annotations, security, module services, module migration) so you might want to wait for our second book to come out.