Use Google Books to read the table of contents and verify if the topics match your syllabus, but do not expect to read the whole book there for free.
: Deep dives into file permissions, directory structures, and links (hard and symbolic).
The Internet Archive (archive.org) often has a copy of Unix: Concepts and Applications . You can "borrow" it for 1 hour or 14 days as a scanned digital loan. This is legal and gives you a page-by-page PDF-like experience in your browser.