There's also an agentquery.com and querytracker.com, which are very good sites for agents.
Do not write the whole book before getting an agent / publisher! Non fiction sells based on query, a proposal (which is quite involved and detailed), and the first 3 chapters.
Also, you need to be flexible in terms of layout, structure, etc, because from what I know, these are the elements most often played with by your editor.
it might also be a good idea to workshop your query until it is the best you can imagine, because agents get so many of them and yours needs to stand out. Though I've heard of stories of really "bad" queries working well, because an agent just loved the idea!
Good luck with your query!