Yes,
you have understood well, O'Reilly put more than 50 books for free on their openbook section, some of this books are old but they are yet useful, some other books are newer and very useful.
Some of them are in pdf format, others in html, others in wiki….
My favourites are:
- Free as in Freedom
- Linux Device Drivers, 3rd Edition
- Understanding Open Source and Free Software Licensing
- We the Media
- Linux Device Drivers, 2nd Edition
- Using Samba
And yours?
View the full list here: http://oreilly.com/openbook/