1) Tree Metaphor

Imagine you plant a tree. In ten years, that tree can give you $100 in lamber and in 100 years, $ 1000.
Now from the financial perspective.

With a currency that yields 5% interest, $100 in ten years are equivalent to $ 61.39 today. And $1000 in 100 years are equivalent to $ 7.60 today.

If the currency has 5% demurrage, $100 in ten years are equivalent to $ 167.02 today. And $1000 in 100 years are equivalent to $ 168,903.82 today.

With interest, the same stuff in the future is valued less than today. With demurrage, the same stuff in the future is valued more than today.

This proves that the structure of money has an impact in our way to value things over time.

2) Isn't demurrage equivalent to inflation?

No. Their impact on the gross interest is the opposite. Demurrage removes the privilege that lenders have over borrowers and the demurrage is substracted from the basic interest.
With inflation, the money holder could just buy things and sell them later at a higher price. That has to be taken into account when negotiating the interest.
This is added to the gross interest in the form of inflation premium (Hausse-premium in the text).
The reason why we have low interest with inflation today is the way the inflation is created.
Central banks monetize debt by buying bonds and giving cheap loans to banks. This way, when the real savers (not the central bank) go to the financial market they find that some borrowers (the banks and the governments) have already obtained its funds with cheap loans and they have to lower their prices (their interest) to meet the demand that the central bank has decreased.
Real savings have to be balanced with investments and that's in my opinion the most important lesson from the austrian school. But that's not incompatible with demurrage.
They found out that increasing the money supply doesn't solve the problems of deflation, just postpone and aggravate them.
But with demurrage you incentive money circulation without increasing the money supply.
 
3) Money is just another commodity.

It shouldn't be. Money is a symbol of value. If the demand for sugar is increased, producers will increase the supply until the price gets just a little above production cost again.
If the demand for sugar decreases, the supply will be reduced until the production costs are are below the selling price again.
But if the demand for money increases, should we increase the supply? If it the demand decreases, who's money are we going to destroy?
Also, who are the money producers? Must the production costs of money be related to its value in the market?

If you want to read the book, you can skip the first two parts. They're about another concept, free-land, but I'm not sure removing the property of land is a good idea.
I'm happy to answer your questions, but I'm sure you will understand the concept of free-money better if you read the book.  
Also, we should probably discuss this in the freicoin thread.