• Martin Fowler’s definition of integration testing: https://martinfowler.com/bliki/IntegrationTest.html.
• Ham Vocke on the practical testing pyramid: https://martinfowler.com/articles/practical-test-pyramid.html.
• Eugen Kiss’ blog post explaining why he prefers integration testing more than unit testing: https://blog.usejournal.com/lean-testing-or-why-unit-tests-are-worse-than-you-think-b6500139a009.

Further reading:

• Nice explanation of the unit vs integration vs system terminology: Graham, D., Van Veenendaal, E., & Evans, I. (2008). Foundations of software testing: ISTQB certification. Cengage Learning EMEA.
• Pragmatic definition of unit testing: Osherove, R. (2015). The art of unit testing. MITP-Verlags GmbH & Co. KG.
• SOLID principles: Martin, R. C. (2002). Agile software development: principles, patterns, and practices. Prentice Hall.
• Use of mock objects as a way to guide class design: Freeman, S., & Pryce, N. (2009). Growing object-oriented software, guided by tests. Pearson Education.
• Modeling domains: Evans, E. (2004). Domain-driven design: tackling complexity in the heart of software. Addison-Wesley Professional.
• Design patterns: Gamma, E., Helm, R., Johnson, R., & Vlissides, J. (1993, July). Design patterns: Abstraction and reuse of object-oriented design. In European Conference on Object-Oriented Programming (pp. 406-431). Springer, Berlin, Heidelberg.