If you invent a new way to turn grass into
building panels, nobody else is allowed to do that,

Obviously you are not an inventor. In your example the person is protected from
someone using exactly the same method of turning grass into building panels but
if someone makes a minor change in the process they can patent it as the new
and improved grass panel.
It is much like your novel.
Again the basic idea is not protected, only the particular details of the
process to realize the idea.
It is also a lot harder to actually get a patent. That is why so many things
are marked "patent pending". If you actually waited until you got the patent
before you bring the product to market someone will knock you off and saturate
the market before you can stop them.
If you look at the history of great inventions it is never the guy who came up
with the idea who got rich, just the guy who got it to market first.