Splinter of the Mind's Eye occupies a curious place in Star Wars history. Published in early 1978, it is the earliest non-movie novel to be written, and next to the Marvel ongoing series, the second piece of what would later be called “The Expanded Universe.”
Its author, Alan Dean Foster, a newer but already prolific writer by the late 70s, had been contracted for two books. The first was the novelization of Star Wars (ghostwriting as “George Lucas”), which was worked from earlier script and story drafts (which is why Luke is a member of Blue Squadron instead of Red and scenes like Biggs on Tatooine are present but cut from the movie) and Splinter was the second.
On the whole, it operates on a much smaller scale than Star Wars. The bulk of it takes place on a single planet, and the only returning characters are Luke, Leia, Threepio, Artoo & Darth Vader. The reason being that Lucas was hedging his bets in case Star Wars was not massive success and the potential sequel could be adapted from this book on a much, much lower budget. Han Solo & Chewbacca don't even appear because Harrison Ford hadn't signed on for the sequel yet. The movie was a success, so there was no need to go small for the sequel, and so this book occupies its curious niche in Star Wars history.
Despite the smaller scale, the plot ends up being pulpy as hell. Luke & Leia travel to the Circarpous system to recruit for the Alliance when technical troubles cause them to crash land on the swampy planet Mimban. Braving wildlife, they find an Imperial controlled mining town, get captured by the local Captain-Supervisor, Grammel, escape, and join a strange old woman, Halla, on her search for a powerful Force artifact called the Kaiburr Crystal. And then Darth Vader shows up for a showdown in an ancient temple of Totally-Not-Cthulhu.
Its a short read, and moves quickly from situation to situation. Grammel, the primary villain, starts out as an intelligent but brutal bully of a man who's a big fish in his small pond, but as the situation spirals out of his control, he starts losing his cool, and then has his authority pulled away from him when Vader arrives.
Halla is a little bit like Obi-Wan, since Luke looks to her for guidance in the Force, except she's only a minor talent who's really proud of being able to move a few small objects. More interestingly, she's a shady treasure hunter with a cowardly streak, and she gets a fun character ark.
The other two major characters are Hin and Kee, two big Wookiee-like Yuzzem that Luke befriends in prison. They're miners and have a nasty hangover, but they've got no love for the Empire and gladly join up in the search for the crystal.
The crystal itself is in the lost Temple of Pomojema, a forgotten god who's basically Cthulhu, only with fabled healing powers provided by the crystal.
I mean LOOK at it
The book's strengths are the action and the deeply detailed description of Mimban's environment. Mood and atmosphere are strongly established. Vader is used sparingly in the story, but very effectively. Leia uses a lightsaber for the first time in the franchise, though it doesn't really go well for her. The action sequences are frequent and varied, from encounters with wildlife to medium-scale battles between Stormtroopers and Coway natives (who are much more threatening than Ewoks, despite also being fuzzy). Lightsabers are very deadly, and the climactic fight scene goes to some dark places before the resolution.
As for the bad, the small scale does hurt the story next to A New Hope, which is grand in nature. Also, by the end it doesn't really feel like much has happened or that there's much in the way of consequences, partially because a lot of the plot isn't referenced or revisited by other writers. In the grand scheme of things, its a side story, and feels like it.
The romance angle is awkward in a post-Return of the Jedi world, since Luke and Leia are very much attracted to each other. This isn't Foster's fault, since that's how it was in A New Hope, and if Lucas had an idea where that relationship was headed in 1978, Alan Dean Foster sure didn't know. Still, not the author's fault for not knowing what the characterization would be like five years later.
What is Foster's fault is how Luke is somehow a better swimmer than Leia, despite coming from a desert planet. That just doesn't make sense any way you look at it.
On its own, Splinter of the Mind's Eye is a solidly entertaining, reasonably well-written piece of light pulp. It doesn't really fit well into what the franchise would later become and got swept under the rug more often than not by later writers, but Dark Horse Comics adapted the story in the 90s with a bunch of continuity fixes to bring it closer in line with the Expanded Universe proper. It doesn't have a whole lot of substance going on, and its not as grandly ambitious as The Thrawn Trilogy, but I would recommend it as a snapshot of the humble early years of Star Wars.