The libros registro de IVA are the VAT record books where you log every invoice issued, every invoice received, and the investment goods of your business; the figures of the quarterly VAT return come straight from them. They are mandatory for nearly all self-employed workers and companies that charge VAT, also outside the SII real-time system, and the tax agency can request them in any review.
There is no rigid official format: a spreadsheet or invoicing software is fine, as long as the books contain the data required by the VAT regulations and match what you filed. kontora builds them automatically from your invoices and expenses and reconciles them with the draft of Modelo 303.
Rather have this calculated for you?
kontora generates your tax forms box by box, tells you how much to set aside and reminds you before every deadline.