For your RC book online download, you need to follow the steps mentioned below:
Visit https://digilocker.gov.in/ and register yourself.
Under the tab ‘Issued Documents’, select Ministry of Road Transport and Highways
You will be redirected to a new screen, where you need to select ‘Registration of Vehicles’
You will then be prompted to enter your Registration number and your Chassis number.
On submission of the above details, you will then get the RC, which you can download in different file formats under the ‘Issued Documents’ tab.