I was a big fan of this author back in the day, but his works are largely outdated in this day and age of vastly improved resources on both linguistics and foreign languages.
For readers interested in the field of linguistics, there are several better books. I can recommend, among others, _Saussure for Beginners_ from the Writers and Readers "For Beginners" series as well as Fromkin, Rodman and Hyams's _An Introduction to Language_. The latter has gone through many revisions over the years, though, since I used it in my introductory linguistics course in college back in the 1980s, so I can't vouch for the current edition. What I always liked about the version I used was the wide variety of interesting and often humorous examples used to illustrate various linguistic principles.
The problem with Pei's book is just that it's dry and lacks the fun/interesting elements which more recent works bring to the field.
A great introduction to a complicated (especially today) field. Mario Pei is a superb, lucid writer and a pleasure to read. He was one of the world's foremost language experts and the basic definitions and explanations in this book are still useful.