Somebody wrote "Not to sound rude, but how can one judge what idea one needs to see as 'important' while skimming through the book if they have no clue of what the book is about?" and then deleted it. I'd still like to reply because it's a good question. Here goes...

That's a perfectly good question. I used to think that skimming was worthless, only something that people who didn't actually care about information did.

But if you think about how you are constantly judging the value of information everywhere throughout the entire day then you can see how we have to make some filtering decisions.

Systematic skimming is just another filter. If I'm going to dedicate 10 hours to reading the book carefully it makes sense to get an overview of the ground first. If the books I'm reading are from Alan Kay's list of greatest books you should read then I'm going to not worry so much about the filtering aspect of the skimming. If it's a shitty book with a catchy title in the business books section of the library I'm going to be much more skeptical and the filtering part of skimming is essential to not wasting time on crap.

Judge a book by its cover. Then judge it by its table of contents. Then judge it by some passages you read that looked interesting. Don't commit to a book until you've gotten a better feel for what's inside.