I've seen all of these episodes of course, so you might wonder the point of reading them. While adaptations of books to film are often lacking, primarily because of all that has to be left behind and the penchant of Hollywood for re-writing the source material, I have often found novelizations of movies to be an improvement. The written word has an unlimited special effects budget for one thing, and the writers of adaptations usually fill in the gaps of characterization in interesting ways.
The fun of this book is that it predates the fan-boy "canon" of modern Star Trek, where everything has been organized, classified and filled in. In 1968, Star Trek was still something of an unknown quantity, much as Star Wars was in the late 1970's, when we had only seen one film and were still able to fill in the details with our own young imaginations.
Since it predates canon, we discover some interesting details that might be fun to explore in a role-playing game set in a more pulp sci-fi Star Trek setting. A couple that spring to mind are the description of the gorn in "Arena" -
"The first thing he saw was the Gorn. It was a biped, a reptile, a lizard that walked like a man. It stood about six feet four, with tremendous musculature, dully gleaming skin, a ridge of hard plate running down its back, and a strong, thick tail. The tail did not look prehensile; rather, it seemed to be a balancing organ, suggesting that the creature could run very fast indeed if it wished. The head was equipped with two tiny earholes and a wide mouth full of sharp teeth."
Not terribly different from what we saw on screen, except for the "run very fast indeed". The adaptation gives you a much better sense for the fact that Kirk was supposed to be completely outmatched physically by the gorn, and that his only hope for winning was to use his brain.
In "Errand of Mercy", we learn about the Klingons -
"The Klingons were hard-faced, hard-muscled men, originally of Oriental stock."Surprise - the Klingons are humans, though obviously separated from the rest of humanity at some point in the past and thus of slightly different stock, larger and more militaristic. In effect, hobgoblins to the Federation's humans.
Kirk comes off as more of a Flash Gordon-type in the book and Spock is more half-human than Nimoy portrayed him. The book contains other little notions that might make old Trek fans see the series in a new light, and for that reason the book is worth reading. Understand, it is a thin volume (112 pages) and a very quick read, but since I picked it up for $2.80 at a used book store, I would say I got my money's worth.