Publishers publish textbooks to meet the standards of the largest textbook buying states: California and Texas. There is no approval process prior to publication, but books are adopted for use on a state-by-state basis. Textbooks themselves are written by committee, which is why they are so bland.
Anyone can write a biography. Getting published is another thing. Most published authors have an agent who will pitch an idea to a publisher, and often negotiate an advance for the author to live on. The Catch-22 is that you need an agent to get published, but if you haven't been published it's nearly impossible to get an agent. An editor will oversee the process from conception to publication. Obviously, some books are fact-checked more thoroughly than others, although in recent years, some of the most renowned biographers, like Doris Kearns Goodwin and the late Stephen Ambrose, have had to deal with allegations of plagiarism.
