Is it harder to write the first book as opposed to the second (or third, or fourth) or does it depend on the content of what you are writing?
It depends on the content, without a doubt. Each book you write has its own challenges, breezes and surprises; it's all these factors combined that really makes writing fun! If writing a novel got easier, what would be a writer's impetus to keep going? If writing a novel ever became so easy it was boring, I would stop writing.
My third novel, a WIP called 'GLOW,' is harder to write based on the content -- it's paranormal chick lit/love story. But the challenge it presents is what keeps me interested in the story, engaged in the relationship!
My first book, 'Manas,' was fun to write if only because of the sheer effort of writing and new found joy of the project. The second book, 'Arcane,' was brilliant in that I had so much more fun really knowing what I was doing and looking for in a particular scene. The third is great because my characters are so vivid -- they're like a big bunch of girlfriends! My novella rocks my world and is totally scary ...
So, I'm going to stick with content for this answer. The meat of the story is where the challenge and enjoyment of writing live for me, and every single story has a new set of barriers to overcome and treasures to unearth.