For me it was "J2EE Development without EJB" by Rod Johnson / Jurgen Holler, published in 2004. In the late 90's and early 2000's Sun insisted you could build industrial scale web applications with Java, but only with complicated frameworks running on heavy application servers that cost tons of money.

Rod Johnson showed how you could code in a lightweight, highly productive fashion with Pojos (Plain old Java Objects) and still scale. The principles were generic but became the foundations of the Spring framework that took over the ecosystem.

"Hibernate in Action" by Gavin King is another that came out around the same time. Again, a book on generic ORM principles, focused on a particular library. It also rejected the old orthodoxy and became the new standard.