Short answers, pulled from the story.
Salt's 1892 book Animals' Rights: Considered in Relation to Social Progress is credited by scholar Keith Tester as the first work to argue explicitly for animal rights rather than simply for better animal welfare. Salt held that animals possess individuality and are therefore entitled to a form of restricted freedom that constitutes rights, not just protections.
Gandhi encountered Salt's A Plea for Vegetarianism in a London vegetarian restaurant and was deeply impressed. Salt's ethical argument helped Gandhi shift from a religiously motivated vegetarianism to an explicitly moral one. Salt also introduced Gandhi to the works of Henry David Thoreau. The two men shared a platform at a 1931 London Vegetarian Society meeting, where Gandhi delivered a speech titled 'The Moral Basis of Vegetarianism.'
Salt co-founded the Humanitarian League in 1891 on the conviction that evolutionary biology had dismantled old distinctions between races, classes, and species. Based at Chancery Lane from 1897 to 1919, it campaigned against corporal punishment and blood sports, published the journal Humanity, and hosted the first National Humanitarian Conference. It dissolved when Salt stepped down in 1919; former members founded the League Against Cruel Sports in 1924.
Salt taught classics at Eton from 1875 until 1884. He left to devote himself to writing and humanitarian reform, moving with his wife to a cottage at Tilford in Surrey where they grew their own vegetables and lived on a small accumulated pension. The move reflected his deepening moral rejection of the social order that institutions like Eton represented.
The book was reissued in 1980. Philosopher Peter Singer wrote the preface, calling it the best work on animal rights from the 18th and 19th centuries and noting how Salt had anticipated many of the central debates in the contemporary animal rights movement.
Salt wrote almost 40 books during his lifetime. They ranged across vegetarianism, animal rights, literary criticism, biography, classical scholarship, nature conservation, and political essays. His subjects included Henry David Thoreau, Percy Bysshe Shelley, and Richard Jefferies, among others.