I'll point out that an affiliation with some open source group (Perl Mongers included) will usually get you a 50% discount on OReilly's ebooks. Once you've purchased that ebook, OReilly will allow you to download it in a variety of formats. HTML is generally not one of them though.

OReilly often offers free trials of their Safari service. You might give Safari a try and see if it's a good fit. If not, you just cancel before there's a charge.

Also, check eBay for used "Bookshelf" CD's (not copies, but originals, used). The Bookshelf CD's came with HTML format. As an example, an ebay search for "Oreilly Bookshelf" found me an auction that has an original "version 4.0" of the Perl Bookshelf for $5.00. This set contains six books.

Even if you can't find a legal source for free HTML versions of out-of-print OReilly books, there are several legal "low cost", or "no cost for awhile" options: OS discounts, Safari trial, eBay, public library, even trade with friends.

Last year, I heard an interview with O'Reilly on the radio and he said that they don't care if people download their older books, as it helped to spread knowledge and the O'Reilly brand name. That is why they don't take legal action to block these sites. But, I speak from memory, and may be wrong.

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I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh

My memory only contains the bitter feeling after lots of downvotes I got as a newbie here after linking to the very same site. I also read somewhere O'Reilly tried to talk to the owners of the site, but without any effect (source forgotten).
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

.ua is Ukraine. I doubt O'Reilly can take any legal action against sites there.

Having said that, I must admit that I did download the full shebang a number of years ago. And, later, I did buy about a dozen of those books in the collection that I found useful, because a dead-tree volume is very useful and practical when you are using the book very commonly. At least, I knew before buying it about the book content, and I knew it was going to be useful to me.

I do not know about O'Reilly's official position on the subject, but I know for a fact that many publishing houses specialized in highly technical content now prefer to give access to free PDF or HTML versions of their publications (either officially by releasing publicly the content, or less clearly, by selling CD's that they know will be copied), because they've found that giving free access to electronic versions of their best publications actually enhances sales of paper versions of the same. Now, of course, each case is specific.