The Nebula Award is given by the Science Fiction and Fantasy Writers of America — a membership organization of working professionals — which means it is voted on by the people who write the books. This creates a different signal from the Hugo, which is voted on by fans. Writers tend to notice craft problems that readers overlook, and they tend to recognize technical ambition even when it doesn't produce the most satisfying reading experience. The Nebula list skews toward prose quality and structural innovation in ways that the Hugo doesn't always.
The early Nebula winners established both the prize's strengths and its occasional tendency toward insider preference. Frank Herbert's Dune won both the Hugo and the inaugural Nebula in 1966, which tells you something about that book's position in the field. But in 1970, the Nebula went to Larry Niven's Ringworld while the Hugo went to Larry Niven's Ringworld as well — the two awards converging on the same novel for different reasons. The Nebula was recognizing the engineering ambition; the Hugo was recognizing the entertainment value. The two things happened to live in the same book that year.
The most important early Nebula win was Daniel Keyes's Flowers for Algernon in the short fiction category in 1960, later expanded into the 1966 Nebula-winning novel. Keyes's story — told through the diary entries of a man whose intelligence is surgically enhanced, then declines — is a remarkable piece of narrative construction: the grammar and vocabulary of the entries change as Charlie's intelligence rises and falls, so the reader experiences his mental state directly rather than being told about it. It is the kind of formal decision that writers notice immediately, which explains why it won the writer-voted prize.
Ursula K. Le Guin's The Left Hand of Darkness won both the Hugo and the Nebula in 1970, a double that she repeated with The Dispossessed in 1975. Le Guin is the central figure in both prize histories because she was doing what neither award had consistently rewarded before her: using the genre's speculative tools for genuine anthropological and philosophical inquiry rather than adventure or extrapolation. Her influence on the Nebula's aesthetic in the decades following her wins is visible in what the award found worth recognizing: work that used the genre's freedom to ask questions about human nature that more realistic fiction couldn't access.
Gödel, Escher, Bach: An Eternal Golden Braid by Douglas Hofstadter is not eligible for the Nebula — it is nonfiction — but it represents the kind of consciousness-about-consciousness inquiry that Nebula-winning SF keeps returning to. Hofstadter's central argument, that self-reference and strange loops are the mechanism by which meaning emerges from symbol manipulation, is the same question that animates Ted Chiang's Nebula-winning stories about language, thought, and perception. Writers read Hofstadter because he is working through the same problems they are, in a different form.
Ted Chiang is the figure in contemporary SF who has won the most Nebulas in short fiction, and his work demonstrates what the writer-voted award finds most valuable: prose that is calibrated to the idea, not just competent. Chiang's sentences don't waste words. His stories are built around a single speculative premise that he follows with absolute logical rigor — Story of Your Life (the basis for the film Arrival) asks what it would mean to perceive time non-linearly, and it works through the implications with the care of a philosopher. Writers recognize that kind of care because they know how easy it is to gesture at an idea rather than inhabit it.
The Nebula's record over sixty years is a reliable guide to where the field's most technically accomplished practitioners thought the interesting work was being done. It doesn't always align with popular taste — some of its winners are more admired than loved — but that is precisely its value as a reading guide. The Hugo tells you what engaged readers valued; the Nebula tells you what the people who spend their working lives constructing these books thought was done especially well.