I'll second what Julie, Harold and Tami say. All my books are WFH ... educational pubs (Capstone, Macmillan-McGraw-Hill, Perfection Learning, etc.) and they assign the topics based on my interests expressed originally (math, science, nature). But if you are pitching your own ideas and have complete manuscripts, submit to the trade market. Study the nonfiction books that are sold in regular stores (not necessarily just in schools). INK is a great site.
Good luck, Rani.
Vijaya