How to write a library

[Matt]

I am trying to write a graphics engine for my university capstone 
project, and really wanted to give it a try in D, as both a 
talking point, and because I love the language. I'm using dub to 
build the library, and the demo application that'll use it.

However, I've come across a problem. In C/C++, when you build a 
library, you compile and link the source, then provide the header 
files for the library user to include. I have built the library, 
but what is the D equivalent to header files, and what do I have 
to do to prepare and use my library in another project?