The number one thing to consider before you begin writing is where will you sell your books and who will buy them?
If you are writing a non-fiction piece, you have a lot of niche markets open to you beyond the bookstore!
You could have a large organization purchase your book and implement it into their curriculum or make it available for their members. You could have specialty stores that will carry your books. It could become required reading for a school. You could find your niche audience online, too.
There are so many possibilities - but you have to plan ahead. Don't write the book first and then wonder who on earth will purchase it!