The basic and only requirement to access any Zoho service is a Zoho account. Setting up a Zoho account takes only a minute.
Zoho Books was introduced for the Google apps marketplace at launch. This means, you can access Zoho Books on a single sign on mode from your Google apps account; import your Google users and contacts into Zoho Books and use the Zoho Books contextual gadget for Gmail. Once you have Zoho Books installed to your Google apps domain, you wouldn't need to sign in to Zoho Books and Google. Sign in to your Google account and access Zoho Books from the universal navigation. See the steps below to know what we are talking about: