We automatically send invoices with every purchase. (We send an HTML receipt and a PDF invoice.) You can click a button in our receipt email to go to our website where you can add custom information to the invoice, which can include your firm's name and address. If you provide your EU VAT number when purchasing it is added to the invoice.
To request a new receipt for any Leanpub purchase you've made, go to your 'Purchases' page at https://leanpub.com/dashboard/purchases. (If you're not logged in automatically, please read the next paragraph!) After that you can click on the book you want a receipt for and then click 'Email Me a Receipt' at the bottom right of the box on the right. You can add custom information to your receipt from there.
If you're new to Leanpub, here's how this works: we automatically create an account for you when you purchase your first Leanpub book. We do this so that you can always access the latest version of each book. If you haven't set a password for your account, you can do this using the 'Reset Password' link on our Login page (https://leanpub.com/login). Once you sign in, you'll be able to go to your 'Purchases' page.