I have a wonderful writing mentor, Nancy I. Sanders, who taught me to query a publisher before writing a book. And believe me, it works! I just finished writing a 10 chapter work-for-hire book for Legacy Press/Rainbow Publishers, with a topic I had queried them almost two years ago. This means I signed the contract BEFORE writing the book! Querying saves time on our part because we’re not writing an unwanted manuscript.
If you’ve never done this before, give it a try. Find a publisher that you’re interested in. Study their web site and list of books. Try to find a “hole” in their list and offer to write those topics. For example, if a publisher has a series on grasslands, tundra, and forests, you could offer to write books on the other ecosystems of ocean, rainforests, desert, …
You can send a query email to the editor.
I hope it works!