Ask HN: Good books on scalability

Is there any good book that talks about scalable web application? By scalability I mean not just scaling in terms of infrastructure but also design.

We want to build something similar to JIRA that will allow it's users to make things as configurable as possible. Also, it's important to allow 3rd party developers to develop things on top of our platform.

I'm looking for some books or resources that can help us build a perfectly modular and scalable/extendable codebase.