In general, you need to be careful - TOS Android Market usually requires that you use the Google payment processing settings to pay users (see "Paid and free applications").
However, they name two exceptions, one of which, apparently, applies in your case - “Where is the payment for digital content or goods that can be used outside the application itself (for example, buying songs that can be played on other music players). " If your application sells e-books in the form of standard files (for example, epub or pdf), you should be clear.
source share