Well you can easily download or read online the NCERT e-books of any class from the official website of NCERT  ( http://ncert.nic.in/ebooks.html). Also, you can download the official NCERT e-books app from the Google play store on Andriod and Apple Store on ios. Also, there are a number of other apps available on the play store from where you can download the NCERT books but I would recommend you to download the official app of NCERT.