This book is definitely geared toward the general reader. It is simple, short, organized, and accessible. After presenting a short bio of each philosopher and describing his thought, Johnston poses a few questions for the reader to ponder, interacting with the philosopher and his philosophy. It is a better book for someone with little or no exposure to philosophy than for one with previous study in the area. Overall, though, it's just not that great of an intro to philosophy, in my opinion.
So as not to leave you without recourse however, if I were suggesting an excellent basic summarization of influential western philosophers, I would point folks to The Journey of Discovery by William Lawhead. The only difference is that Lawhead's book is a basic textbook-- it's longer and covers many more philosophers. However, I have never encountered anyone or anything that distills robust philosophy down to a few accessible pages and makes more connections among the projects of various philosophers than does Lawhead. Even if you picked the same philosophers from Lawhead's book as Johnston covers, I think your time would be much better spent, and more enjoyably spent as well.
ok, back to Johnston's book. In response to some other reviewers' queries, I can assure you that Marx and Derrida are philosophers and influential ones at that. Yes, Marx has had immeasurable impact on economic and political theory and praxis, which most philosophers have not. However, philosophy has shaped most political thought, and political philosophy is a sub-area. Derrida is certainly a well-known literary critic, famous in what is known as "continental" (often French and German) philosophy. However, because of their far-reaching impact throughout philosophy, both of these men are fine contributions to this book given its scope and purpose.
I'm actually a bit more surprised he chose to include Spinoza, but it's probably due to the unique though of Spinoza and Johnston's attempt at diversity. And of course Hegel is noticeably absent, but he inspired Marx's thought, and perhaps Johnston discusses Hegel some in the chapter on Marx. I don't recall specifically at this point. If Johnston delved more into 20th century philosophy, he would probably begin to include women, but given the history of western philosophy, women are not featured previous to the 20th century. And this very clearly is a history of western philosophy -- perhaps that would have been a more appropriate title.