Having read Carl Sagan's Cosmos in my youth, I always feel guilty re-reading a book:
“If I finish a book a week, I will read only a few thousand books in my lifetime, about a tenth of a percent of the contents of the greatest libraries of our time. The trick is to know which books to read.”
― Carl Sagan, Cosmos
Don't feel guilty, Sagan's quantitative approach is a teribly shallow view of reading. If you feel you want to read something again it's because you expect to get something out of it - perhaps to refresh your memory, perhaps to pay more attention to the subtext of the work, perhaps to study the author's literary or rhetorical techniques. You wouldn't assume that you had learned everything about a complex musical composition or painting from single listening or viewing, why assume you've learned everything worth knowing from a single reading of a book?
The only reading I ever feel guilty about is my aversion to leaving a book unfinished. I'm pretty good at picking what to read, but about once every year or two I encounter some real stinker that is a literal waste of my time, and I feel a bit annoyed with myself for plowing through to the end even though I have long ceased to expect any literary or intellectual payoff.
"A book is made from a tree. It is an assemblage of flat, flexible parts (still called 'leaves') imprinted with dark pigmented squiggles. One glance at it and you hear the voice of another person - perhaps someone dead for thousands of years. Across the millennia, the author is speaking, clearly and silently, inside your head, directly to you. Writing is perhaps the greatest of human inventions, binding together people, citizens of distant epochs, who never knew one another. Books break the shackles of time, proof that humans can work magic."
“If I finish a book a week, I will read only a few thousand books in my lifetime, about a tenth of a percent of the contents of the greatest libraries of our time. The trick is to know which books to read.” ― Carl Sagan, Cosmos