Across the Atlantic JK Rowling has won her court case to prevent a publisher issuing a Harry Potter lexicon. The author has been happy to allow the lexicon project online where it existed as a non-profit reference work about the seven-book series. But Rowling drew the line at it becoming a profit-making book, and the judge agreed. This was a classic argument about 'fair use', the dividing line between critical analysis and going too far.
Loosely put, there's a legal basis for authors quoting from the work of others - but only up to a point. You should fully acknowledge your sources. You mustn't borrow too heavily - about 250 words is a decent rule of thumb. And you should be quoting in the context of a critical analysis, where the substantial content comes for your own study of the chosen subject. But there are plenty of gray areas that smudge the boundaries of what's legal, moral or ethical.
The vast majority of my printed work has been licensed media tie-in books - original novels based on existing creations [Doctor Who, A Nightmare on Elm Street, Judge Dredd] and non-fiction tomes [Thrill-Power Overload, my authorised history of iconic British comic 2000 AD]. But I've also dabbled with independent reference books like Starring Michael Caine, which examined 80 films featuring the British actor without getting his input.
Even when you have a sound legal basis for including brief extracts from other writers in the context of a critical analysis, you still need to tread carefully. For example, there's an online stramash at the moment about a new sci-fi programme guide that includes extracts from fan reviews published on LiveJournal. The author uses these to demonstrate fan reaction to different episodes of the show. Some fans are flattered, others are infuriated.
Legally, it may well be that anything published online is ripe for quoting in the context of a critical analysis. [I used a tiny amount of online material in my 2000 AD book to help tell a fuller story.] The author who quotes is under no obligation to seek permission from those they quote, though there is a courtesy issue. When you have a hundred or more sources in a book, seeking permissions from everyone quoting is close to impossible.
I've seen my own words borrowed from online postings for use elsewhere - quotes from blog entries, twitter comments, even facebook status updates. You can take umbrage about that, or you can recognise that if you want something to remain private, simply don't post it online in a public place. Thanks to search engines, most anything you type online can be found and recycled.