This book and Chamber's Etymological English Dictionary are my two favorite but little-known word reference works. The main difference between this book and Roget's Thesaurus is it focuses more precisely on the actual meanings of the words. A thesaurus will give you plenty of words with similar or nearly similar meanings, grouped by general idea or concept. But most of the words are not exactly synonymous. If you need that, this book will come to your rescue. Another nice feature is, as another reviewer here pointed out, it will even give you examples of words with very similar meanings, but with enough description and definition so you can distinguish among otherwise seemingly identical words and synonyms.
It's also in some ways easier to use than a typical thesaurus, since if you just need a different word and can't recall one, it's easiest to just look it up and get a quick alternative, instead of going through all the extra words you'd get in a thesaurus, but which aren't exactly what you want. Overall, a fine reference work and one that deserves to be better known.