John Addington Symonds - Studies of the Greek Poets (1873)
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Symonds may be best remembered today for his Renaissance histories (though I earlier posted his medieval anthology Wine, Women and Song). This one, however, is "by far the most important book on classical literature of which most professional classicists today have never heard" (Gideon Nisbet Greek Epigram in Reception p11). I'm not a professional classicist and probably few of the blog's readers either but this appears to be a readable, personal and deeply informed work. (So far I've only gone through bits and pieces.)
The book also turns out to have been somewhat controversial at the time, partly due to dealing fairly directly with homosexuality but also for suggesting Greek ethics might be superior to Judeo-Christian. No surprise then that Symonds lost a job because of it or that this was a favorite book of Oscar Wilde when he was at university. (See Thomas Wright Built of Books: How Reading Defined the Life of Oscar Wilde.)
For an example, the chapter Women of Homer opens with "Helen of Troy is one of those ideal creatures of the fancy over which time, space, and circumstance, and moral probability, exert no sway." (I particularly like "moral probability" which seems almost like an afterthought but nails the odd function of Helen in The Iliad.) Symonds then discusses her history, who her parents might have been, and how the dramatists handled her character. Rather than the rehash of secondary sources common to so many overviews this is dense and insightful, reminding me somewhat of Robert Graves book on Greek myths.
The book is also remarkably thorough, not only covering the familiar high points but the Greek Anthology, early mythology, Empedocles, Gnomic poets, and many fragmentary works. (In fact, I first heard about this book in an essay on Herodas whose dialogues were discovered in 1891 and Symonds was one of the first to examine in any depth.)
I've linked to the 1893 third (and final) edition because it includes more material though it's not exactly what Wilde read.