Congratulations! Hopefully you should now be more familiar with your Kali Linux system and you should not be afraid of using it for any experiment that you can think of. You have discovered its most interesting features but you also know its limits and various ways to work around those limitations.
If you have not put all features into practice, keep this book around for reference purposes and refresh your memory when you are about to try a new feature. Remember that there is nothing better than practice (and perseverance) to develop new skills. Try Harder, as the Offensive Security trainers keep repeating.
With a constantly-changing distribution like kali-rolling, some parts of the book will necessarily become obsolete. We will do our best to keep it up to date (at least for the online version) but for most parts we tried to provide generic explanations that should be useful for a long time to come.
That said, you should be ready to embrace changes and to find out solutions to any problem that might pop up. With the better understanding of Kali Linux and its relationship to Debian, you can rely on both the Kali and Debian communities and their numerous resources (bug trackers, forums, mailing lists, etc.) when you are getting stuck.
Don't be afraid to file bugs (see Section 6.3, "Filing a Good Bug Report")! If you are like me, by the time you have completed the steps involved in filing a good bug report (and it takes some time), you will have solved the problem or at least found a good work-around. And by actually filing the bug, you will be helping others who are affected by the issue.