As a writer, I simply cannot be without a first-rate thesaurus -- on Thesaurus Day or any other day. I have a number of thesauruses which I often refer to. The one on the left and the one on the bottom are the ones I use the most.
In case you've not yet used a thesaurus, it is a reference tool designed to help find the appropriate word to use in the context you have in mind. A word can have many synonyms, words which have similar meanings to each other, but those words can't always substitute for each other in every context. For example, I might look outside on a cloudy day and say it's a dreary day out. Although uneventful and pedestrian are both synonyms for dreary, neither could substitute for dreary in this sentence, since they have different shades of meaning.
Most dictionaries will give you a couple of synonyms and antonyms for a word, in addition to its definition. A thesaurus may skip the definition completely and just list words with similar meanings. Words may be listed in alphabetical order, as in the Oxford American Writer's Thesaurus, or they may be arranged by numerical classifications, as they were in the Roget's Thesaurus of my teen years. It was hard to use, so I was delighted when new thesauruses began to appear in dictionary form. Now the newer editions of Roget's Thesaurus also come in dictionary form. I have not used one to see how it compares to the book below, but I'm guessing it is comparable and it's just a matter of taste which one you choose.
One book I'm eager to get is Roget and His Thesaurus which I just discovered listed on the Roget's Thesaurus page linked to above. For readers in second grade or above, it tells the story of Peter Roget and how his thesaurus came to be written. It was the first thesaurus ever, his own invention, a result of loving to make lists of words. I had no idea until tonight he had also invented the slide rule.
Each thesaurus has its unique features. I'm currently looking at my Oxford American Writer's Thesaurus. I happened to land on the word dress. It is shown as all its parts of speech. Dress can be used as a noun and as a verb. Each part of speech has a list of related words used in sentences so you can see the differences in meaning and pick the right word. If a word is also commonly used in specific phrases with their own meanings, perhaps idiomatic, they will also be shown. Examples would be dress up and dress down. There is also a helpful word bank listing many different names for types of dresses, eg. ballgown, chemise, caftan, housedress, etc.
Besides the thesaurus itself, there is a list of the word banks found near various words, so you can know where to find them. The list of types of dresses referred to above is a word bank associated with dress. Another list found at the beginning lists word spectrums, e.g. fat/thin. It is found attached to fat. It lists all the words describing the degrees between fat and thin in order. Example: fat, obese, corpulent, Falstaffian, rotund and so on, ending with anorexic, cadaverous, gracile, macilent, thin. Both these aids help you find words in useful groups so that you can choose the exact word you need.
At the back of the book is a reference section containing a language guide, a list of proofreader's marks, and some writing prompts. The language guide offers quick help with grammar, spelling, capitalization, and punctuation. These references are a great help to me as I write, since I don't have to jump up to consult another book if I run into a sticky punctuation or grammar question.
A thesaurus is most useful to someone who already has a well-developed vocabulary. Most people know and understand more words than they may use in conversation. When they write, one of the words they use most often might be the first one they think of, but then they realize that word may not be the word that fits best in the sentence they are writing. It may not have the exact meaning they want to convey . That's when one will pick up the thesaurus to get more ideas. One still needs to be able to recognize from the list of words in the thesaurus which word will be most appropriate. That's why it's helpful to also have a dictionary close at hand to help you understand the shades of meaning in listed words you may not know.
I recommend that if you need a thesaurus, you get it in real book form, preferably a hardbound book, since you will use it frequently. I would not recommend a Kindle edition. A Kindle is fine for novels and books you don't need to thumb through, but Kindle navigation is not ideal for skipping back and forth between pages. At least that's been my experience. Your book will be around for a lifetime if you take care of it, and no one will be able to revise it without your knowledge or approval as might happen with an E-book.
If you are a writer or a student who must write often, a thesaurus is an essential tool. Invest in a complete one, such as the Oxford American Writer's Thesaurus I described above. I reviewed the First Edition published in 2004, but this new Third Edition is supposed to be even more helpful. Cheaper paperbacks offered of other thesauruses only offer a few synonyms per word entry. Those are similar to the thesaurus apps I find free online. Just as a chef finds a quality set of knives essential to his work, a writer will need the best thesaurus he or she can afford. There are plenty to choose from and one of them will be just right for you..