There are many textbook publishers out there. But how many of them do you know are successful? Only a few names are household names in each area of expertise. For example, to some people, Roger Arnold may be a familiar to those who take economics but to others, it is simply a case of 'who?'
If you are thinking about venturing into this area of field, I strongly suggest you think twice about it. Yes, the potential rewards out there are huge. Just imagine every college in US adopting to use your book for just one semester. However, try and look beyond this wall of optimism. Countless people before you have attempted, fail, tried again and failed again.
There are many other ways to earn money through the textbook. You can be a supplier of textbooks, a retailer, or simply try to promote other textbooks to students who know that one textbook isn't going to be adequate. While the grass may be truly green on that side of the pasture, it is almost certainly inaccessible.