Some books and journal articles are released on the web using an open access publishing model. This means that the rights holder has decided to make them freely available for anyone to read. Re-use restrictions are still likely to apply though, so be sure to read any terms and conditions carefully.
This is an example of an open access book: at the very bottom of the page there is a statement that the work is licensed under a Creative Commons licence; by clicking on the icon you will be taken to a page showing the licence terms.