"If nature abhors a vacuum, why did she make so much of it?"
Back in the mid-70s, I picked up Brian Stableford's The Realms of Tartarus - about the underworld of an Earth paved over. I thought it was great, and I still think Stableford is very much underrated. I hope Open Road will reprint a lot of his other books as ebooks.
Man in a Cage was published around the same time as Realms of Tartarus, but it's an entirely different kind of book. Man in a Cage is an intensely psychological study of one tormented, isolated individual. The book is told in three distinct, interwoven parts - standard, real life narrative of Lee's interaction with the space program; semi-autobiographical sections from the perspective of Lee's inner self (or selves); and the surreal travels of what may be Lee's true self, especially as he deals with the impact of faster than light travel. They all blend together toward the end.
The book wasn't at all what I anticipated, but I liked it a lot. At almost every turn, Stableford avoids the easy, obvious approach. The autobiographical segments devolve into discussions of psyche and society. Much of the time, they're interesting, but they do at times drone, and Stableford gets lost in his concept - too much of a good thing, and the book would have been stronger if they'd been cut back. At the same time, they're insightful and often funny. The 'man in a cage' metaphor comes in for heavy use, but it manages the trip from interesting to overdone and back to relevant. The prose is expertly managed, and the characterization (almost all from Lee's perspective in one way or another) is deft.
This is not a quick and easy read (I took a break by zipping through Brandon Sanderson's The Rithmatist), but it is rich and rewarding. While I greatly enjoyed Sanderson's light adventure, Stableford's book is a different pleasure altogether - the kind of SF that makes you think without beating you over the head, and all presented in carefully measured but moving prose.
If you haven't run across Brian Stableford before, this is a great place to start, and I recommend you do so. If you know his writing, it's a good place to continue. Others have tried to capture the perspectives of narrators with their own reality, but few have done it so confidently and compellingly.