The best keywords will be those that are popular search terms (terms that people actually search for), and that clearly describe the purpose and the content of your pages. They will also have to be keywords for which there is relatively low competition.
There is no sense in trying to optimize your pages for super-competitive keywords where you don't stand a chance of ranking well (most one-word keyphrases fall into this category: for example, it would be impossible to rank well for the keyword 'book').  It would be better to target phrases like:
  • 'cook book' - easier but still competitive
  • 'italian cook book' - easier and less competitive
  • 'northern italian cook book' - easier and not as competitive. 
The more specific your keyphrase, the easier it will be for your site to rank well. You should therefore target keyphrases with three or more words to significantly improve your chances.
Choosing the wrong keywords can throw off your entire search engine optimization strategy. It will force you to re-work your pages for new keywords and wait for the search engines to re-crawl your site.  For this reason, a few hours trying to pick the right keywords is time well spent.
To start, take a look at your pages.  Look at them carefully and write down the words and phrases that best define your site. Try to build a list of two or three word phrases. Once you have developed this list of potential keyphrases you are ready for the next step: to analyze the demand and supply for those keyphrases, and choose the best ones (those with good demand and not enough supply).