> ## Documentation Index
> Fetch the complete documentation index at: https://docs.billbooks.com/llms.txt
> Use this file to discover all available pages before exploring further.

# API Reference overview

> How the reference section is organized, and the three small endpoints worth knowing about

The pages under this tab are generated directly from Billbooks' OpenAPI spec, one group per resource. Each shows the exact request and response shape, every field, and every validation rule as they exist in the spec today, this section stays in sync with the spec automatically rather than being hand-written.

Five resources carry real business logic and have their own guides walking through the non-obvious parts: [Invoices](/guides/create-and-send-an-invoice), [Estimates](/guides/create-and-manage-estimates), [Clients](/guides/manage-your-client-list), [Items](/guides/build-your-item-catalog), and [Expenses](/guides/log-expenses-via-api). Start there if you're building something, then use the reference pages for exact field-level detail.

Three more endpoints exist purely to support the five above:

<CardGroup cols={3}>
  <Card title="Reference data" icon="tags" href="/reference/reference-data">
    `GET /taxes` and `GET /expense-categories`, read-only, no pagination. The only way to discover valid `tax_id` and `category_id` values for use elsewhere.
  </Card>

  <Card title="Account" icon="id-badge" href="/reference/account">
    `GET /me`, confirms which organization your key belongs to and the key's own id and name. Never returns the key hash or any other secret.
  </Card>
</CardGroup>

<Tip>
  If you're just getting started, [Authentication](/authentication) and [Quickstart](/quickstart) are a faster on-ramp than browsing the reference cold.
</Tip>