Just over one month ago I reviewed Wordbook, my review of Wordbook was positive and I ended up recommending that you add Wordbook to your iPhone. I standby that recommendation, but as usual, the iPhone application landscape is changing. The popular developers behind the website dictionary.com have just released the iPhone application Dictionary.com for iPhone (free).
Dictionary.com for iPhone has 275,000 definitions and 80,000 synonyms loaded onto your iphone in the 35 mb application. One of the advantages that Dictionary.com has over Wordbook is that you do not need a network connection to access the dictionary data. Dictionary.com for iPhone is no-nonsense, there is a search page for Dictionary and Thesaurus, a Recent screen - where entries are identified as Dictionary, Thesaurus or Word of the Day and a Word of the Day screen. By comparison, Dictionary.com does not have as many extra features as Wordbook, specifically the external links to Wikipedia or the crossword solver to name a few. Dictionary.com requires a network connection for Word of the Day and audio pronunciation. Audio pronunciation on Dictionary.com is vastly superior to pronunciation on Wordbook.
Conclusion, I plan to keep both applications on my iPhone at least for awhile, I like the idea of having all the data on the phone for offline work, and the definition/synonym screen layout and linking is cleaner in Dictionary.com than in the Wordbook application. That being said, the external linking afforded by Wordbook is useful from time to time. Keep them both! Then decide.