Your approach to selling a bunch of "widgets" is typical and it kinda works, sometimes.
Remember that your "niche" is a bunch of do-gooders that are broke or don't care about money. <-- THIS DOES NOT VIOLATE THE BE NICE POLICY, on the forum.
Instead of spending a bunch of money on a marketing company that is going to approach this same group of save the worlders, pull excerpts from the book and build small ads that the affiliates can use.
Find these other niches that have interests in some of your knowledge, but not all of it.
- I want to build an RMH to save on the gas bill.
- She wants to garden some herbs and veg in a mostly concrete yard in the city with neighbors less than 10 feet on either side.
- He wants to collect and use rain water to stick it to the mismanaged utility company and save money on jacked up prices.
Give them solutions to their problems.
You're way smarter than me, so you get the idea.
Build landing pages for these specific groups so that when they click that link Attention Interest Desire Action (AIDA) goes into effect and they buy.
The affiliate idea is the best way, but you need actual affiliate marketers to get it done. Put your book on Clickbank with 50% and someone will start selling it.
There are others, besides Clickbank.
I could go on, but that would mean unintentionally offending people.
You have my info, I have an affiliate number, we have 1 million to move in 6 months.