This is the "swiss-army knife" of english word definition. For me it replaced a pricey brand name dictionary (with a bad UI) and a free thesaurus application. Search is quick and flexible, updating the word list as you enter each letter, the dictionary is purported to have the equivalent of over 1800 book pages of information, this source of the content is Princeton's Wordnet database (a free DB that can be accessed via a web browser, with WordBook you are paying for the application's UI). After 3+ months of using WordBook I have had success with all word searches.
The user interface is straight forward with a content window, linked content is identified as blue text, the font is readable and its size is adjustable via settings. There is a standard iPhone-esque icon bar along the bottom of the screen, giving you one touch access to "Featured" (words of the day), "Favorites" (you can mark any entry as a favorite - viewed here, also you can view your word search history here as well), "Search", "Definition" and More... (Crossword & Anagram dictionaries, Pronunciations table). Once you have a word selected there is a button bar across the top, that provides access to Dictionary, Links, Thesaurus and Web. There is also a sliding tab that opens when touched, giving you the ability to email the definition or use the customizable web links. The web links are a great idea that increases the usefulness of WordBook by extending its feature set via online websites, for example you setup an URL to a medical dictionary, with a quick touch the word you are searching is dropped into the web resource seamlessly inside of WordBook's browser. While WordBook leverages the web to enhance its own content, an internet connection is not required to run WordBook, as 200,000 entries are optimized and loaded onto your iPhone for access anytime your iPhone/Touch is powered up.
In my opinion this $3.99 application holds up well against the $20-30 dictionary applications in the App Store and surpasses them in fuctionality. WordBook is worth checking out! This is a Real World application!