You can find useful books in well-organized format at http://nbcampus.com/students-page/free-books/
Thousands of websites are available to download NCERT /CBSE books. Most of these are funded by online advertisements thus you may be asked for free or paid registration which may be time-consuming. Browsing untrusted website may cause harm to your laptop/mobile by pushing virus in these devices.
Considering these all facts, I will recommend students / scholars to use below mentioned official url to download ncert/cbse books..
Hope it will be very useful and time efficient for you.