I initially read this book about four years ago and it made a lasting impression, so I re-read it again recently and found it just as absorbing.
One thing should be pointed out from the start as one reviewer has noted, this isn't full-on SF. Steer clear if that's what you're looking for (incidentally there are other books in S.F. Masterworks series that are not hard SF e.g. I am Legend). I should also add that I do not personally believe in the existence of powers such as telepathy or ESP! So, to the book itself...
Dying Inside charts the life of 41 year old David Selig and his gift/curse of being able to read people's minds. It explores his struggle for self-understanding and the manner in which his ability both elevates and alienates him from humanity. The story is told through a variety of narrative devices such as the re-reading of old letters, flashbacks to past events, and Selig's present situation as his power begins to ebb away. The themes dealt with in the novel are intensely human and concern love, rejection and acceptance, ageing, and what it means to understand and know others. I found Silverberg's approach to the concept of telepathy to be intensely vivid and convincing, as Selig veers from God-like omnipotence with his power through to being a despairing misfit; all of which is expressed through Selig's day-to-day life and encounters, such as his relationships, work and social identity.
In structure the novel does not follow any real plot and at times it lacks cohesion, but this seems to work in the novel's favour as it mirrors and reflects Selig's character. It also contains some beautiful descriptive writing (particularly toward the end of the novel). To put the novel into context, it was published in 1972 in a decade where the author hit a rich vein of form, with books such as The Man in the Maze and The Stochastic Man.
Overall, on a personal level this remains one of my favourite SF novels. If you're looking for a futuristic storyline, built around science and technology then this won't be for you. But if this and the other reviews intrigue you then I'd certainly recommend you give it a read.