# Billbooks Developers

## Docs

- [Introduction](https://docs.billbooks.com/introduction.md): What the Billbooks API does, who it's for, and how it's organized
- [Authentication](https://docs.billbooks.com/authentication.md): How to create and use an API key
- [Quickstart](https://docs.billbooks.com/quickstart.md): Make your first API call: create a client, then send them an invoice
- [Rate Limits](https://docs.billbooks.com/rate-limits.md): How rate limiting works on this API, and its real limitations
- [Errors](https://docs.billbooks.com/errors.md): The error envelope shape and what each error code means
- [Create and send an invoice](https://docs.billbooks.com/guides/create-and-send-an-invoice.md): The full invoice lifecycle: creating, editing, and the side effects that come with each
- [Manage your client list](https://docs.billbooks.com/guides/manage-your-client-list.md): Listing, filtering, creating, updating, and deleting clients
- [Build your item catalog](https://docs.billbooks.com/guides/build-your-item-catalog.md): Creating catalog items, and how stock tracking actually behaves
- [Log expenses via API](https://docs.billbooks.com/guides/log-expenses-via-api.md): Recording expenses, how invoice-linking works, and a gap you should know about
- [Create and manage estimates](https://docs.billbooks.com/guides/create-and-manage-estimates.md): Estimates are structurally simpler than invoices, and carry no financial side effects
- [API Reference overview](https://docs.billbooks.com/reference/overview.md): How the reference section is organized, and the three small endpoints worth knowing about

## OpenAPI Specs

- [openapi](https://docs.billbooks.com/openapi.yaml)