In order to get oBooks, in Octavo these are exchanged for oCoins. And besides the option of buying the oCoins via credit/debit card, it is also possible to invite your friends via email and get some free ones.
From the main side menu, select My oCoins. Once in this section, tap on Free oCoins and input the user's email to invite.
On the main side menu, select Free oCoins and input your contact's email.
Note: You will get your oCoins after your invitation is accepted (signed up) and this new account is email-verified.