Hi Lee,
I think you could use Overture.com's free keyword search tool to see how many searches there are for "inspirational quotes," "motivational quotes," "religious quotes," and so forth. Then also use the bid tool to see if other sites are paying much for those keywords. If they are NOT, that may mean that, even though lots of people may be searching for those terms, those people may not be willing to pay anything for them. Popular terms aren't always profitable ones.
I believe there are many web sites that already offer free quotations, so it may be a difficult niche for an ebook to make money.
But I'm fairly new to Internet Marketing, so please just take this as my personal opinion, and certainly not "expert advice."