Chapters 20 & 21 as a starting point
Page xxiv (Introduction) describes Chapers 20 & 21 as "great references" for those who have used other operating systems; Chapter 1, Unix Fundamentals begins with UNIX 'history' starting nearly 30 years ago.
Given the title of this book is "beginning" Unix (where 'beginning' is a present participle of the verb to begin), I suggest that the authors should offer two approaches to beginning. Many (if not most) folks are beginning their exploration of UNIX with a starting point in another operating system. For example, as a Macintosh veteran, I found the command and GUI equivalents that begin on page 379 (in Chapter 20) as the perfect spot for me to begin.
I suggest the very first paragraph of Chapter 1 is a perfect spot to offer prospective readers some choices as to where to begin â including Chapters 20 & 21 as options. Given that a major purpose in Chapter 1 is "cover[ing] the fundamental concepts of the basic Unix operating System", you cannot offer a better path into those concepts than to use the bridges built in Chapters 20 & 21 for the millions of users of 'popular' operating systems