Don’t worry about reading it all the way through. Just find something where you say “Aha, I didn’t know you could do this,” then build a small demo and expound on it.
Speaking as someone who learned it 10+ years ago, I think this is the best way–or if not the best way, it left me less frustrated with the parts I didn’t understand yet.
Some sections meant little or nothing to me for a while, until I needed them or wondered “can Inform do that?” or “can Inform do that better?” And yes, it could.
Also, look at source code of works you really like and wonder “How’d they do that?” Source code’s more prolific now, what with GitHub being more prominent. I wound up learning a lot of I6 from Friar Bacon’s Secret.
As for the original question, I’m glad someone else asked it. The white whale in this thread seems within reach. But I might have something smaller to write, too.