Moneybird

Moneybird's API is developer-friendly and well-documented, designed specifically for accounting integration.

It supports both OAuth2 and API keys, with sandbox environments for testing. The RESTful API covers invoicing, contacts, payments, and financial reporting comprehensively. JSON and XML formats are supported, with clear error handling. However, it requires a Moneybird subscription and is primarily focused on small business accounting needs.

It's an excellent choice for Dutch accounting automation, but the learning curve depends on your familiarity with accounting workflows.