For synonyms are not words that have the same meaning.  They are words that have similar meanings.  They may be compared to circles that overlap but do not coincide.  Each embraces a common area, but each embraces also an area peculiar to itself.  Though many words cluster about a given idea, rarely if ever are even two of these words entirely equivalent to each other.  In scope, in suggestion, in emotional nuance, in special usage, or what not, is sure to lurk some denial of perfect correspondence.  And of synonyms, so of antonyms.  Antonyms are words opposite in meaning; but the opposition, for the same reasons as the likeness, is seldom or never absolute.

In your study of synonyms you will find most of the dictionaries previously named of great help.  You may also profitably consult the following books of synonyms (heavy, scholastic works not suited for ordinary use are omitted): 

Books of Plain Synonyms and Antonyms

Edith B. Ordway:  Synonyms and Antonyms.  A compact, practical volume, with antonyms (in italics for contrast) immediately following synonyms.

Louis A. Flemming:  Putnam’s Word Book.  A book of the ordinarily used synonyms of words, with antonyms after some of them, and with lists of associated words wherever these are likely to be useful.

Samuel Fallows:  100,000 Synonyms and Antonyms.  A handy little volume, with useful lists of various kinds in appendices.

Richard Soule:  Dictionary of English Synonyms [revised and enlarged by George H. Howison].  A much larger and more expensive book than the others, and less practical for ordinary use, but fuller in treatment of material, with words of more than one meaning carefully divided into their various senses.

Synonyms with Word Discriminations

George Crabb:  English Synonyms.  A standard volume for over 100 years.  Has close distinctions, but is somewhat scholarly for ordinary use.  Revised edition of 1917, omitting illustrative quotations from literature, not so good as editions before that date.

James C. Fernald:  English Synonyms, Antonyms, and Prepositions.  A pleasing book to read, with much information about the use of words and their shades of meaning (with exercises), also with proper prepositions to follow words.  Material taken from the Standard Dictionary.

Peter Mark Roget:  Thesaurus of English Words and Phrases.  Issued in many editions and revisions.  Words grouped under general ideas.  An excellent book for serious and laborious study, but not for quick use.

How to Acquire Synonyms

The best principle for the extension of one’s mastery of synonyms is the principle already used over and over in this book—­that of proceeding from the known to the unknown.  It is the fundamental principle, indeed, of any kind of successful learning.  We should build on what we have, fit each new piece of material into the structure already erected.  But normally it is our ill fortune to learn through chance rather than through system.  We perceive elucidation here, draw an inference there.  These isolated fragments of knowledge may mislead rather than inform us.