There are a number of good starting points, depending on your background. Note that you can always get back to the first introductory page (and hence the link to this page) by clicking on the title of the corpus in the header above.

What types of searches can you do?	-- Introduction/ five-minute tour (start here) -- Compare to the standard Google Books site -- Comparing American and British English -- Using collocates (nearby words) to see (changes in) the meaning of words -- Basic search syntax (wildcards, lemmas, part of speech, etc) -- Comparing sections of the corpus (i.e. now vs. then) -- Using synonyms as part of your queries
Background information and some limitations	-- n-grams: what are they? -- n-grams: what are some problems/issues? -- n-grams: the "40 token" threshold -- n-grams: limitations for collocates searches
The big picture	-- Future enhancements -- Do I really need such a large corpus? -- I just want to look at data from the last 20-30 years.