For those who care about copyright, that URL is not from O’Reilly; it’s a copy of a book-set that O’Reilly used to distribute via CD-ROM – with a nice user interface that used web technologies (even included a search feature). O’Reilly could make it available for free – as they’ve done so for other books such as Apache Security¹ or Using Samba² – but they still (as is their right) expect you to pay for the sed & awk book³.

¹ https://blog.ivanristic.com/2015/02/apache-security-ten-year...

² https://www.oreilly.com/openbook/samba/book/

³ https://www.oreilly.com/library/view/sed-awk/1565922255/