I already had one Samba book on the shelf (from SAMS) that was little or no help with the connections problems I was having on my network, so I was gunshy about buying another useless one. The O'Reilly online-freebie let me read it to see the quality, and the troubleshooting tree section led me to solving my network problem. Having gotten the benefit of that much, I wanted the book in handheld form to keep handy for reference so I went out and bought it. The sale wouldn't have happened without the freebie.
Here is an alternative unsubstantiated hypothesis: the forces that changed content delivery are going to screw with advertising too, and maybe the two problems are related.
Already demonstrated, methinks, by the Sears-Roebuck catalog, which was a scarce and valued resource in its first century; and Computer Shopper Magazine, which was once about an inch, filled with almost nothing but advertisements, and we valued it for that and read it avidly and respected it as a reference. Now the Sears book isn't exactly the only thing to read by the firelight, and Computer Shopper has faded away almost entirely. For that matter, there used to be a local want-ad rag in each major city (The Recycler, for example, in LA), and now, where they survive, they're struggling.
IMO Advertising is partially subsidized content, differing only in kind, not in class, from created-for-hire and found-as-free content.