I would say the hardest thing about working in a series is remembering what minor characters looks like. I'm like "Is Mike's mom the heavy-set one?" "What was the math teacher's name who just shows up for one page in book #1?" I then have to re-read the first book or just use CTRL+F to get the right bit. That can be time-consuming.
The best part is really knowing the main characters. A lot of my writing is voice-driven, which means that the narrators have a really distinctive storytelling style that is what carries the stories more than super-exciting plot bits and intense melodrama. I work hard to develop a unique voice for each book so it's kind of nice to fall back into a voice I've already created rather than starting from scratch. And I really know the other two main characters well, so dialogue just flows. When I'm teaching writing, people often ask me how to craft dialogue. Sometimes it's because they think I do this well (thank you!) and sometimes I think it's just a common problem for a young writer. Good dialogue is easy to identify but hard to identify what makes it work. The advice I always give is just to spend some time getting to really know your characters. That sounds all touchy-feely but my point is, if you have really thought about who these people are, if you make them very real and very interesting and very full of points of view, and then you turn them loose in a scene where something fascinating is happening, well then it's hard to get them to shut up. Their voices will pour out faster than you can type and it will feel real. I'm not saying it's all easy, but it is fun writing about this group of boys who I know -- and who know each other -- so well.
The title of this 2014 book is SAY IT AIN'T SO. This, yes, is a reference to a (supposed) famous baseball utterance. But it also happens to be relevant to this book because one (or more!) of our main characters is accused of a heinous act. I also think the plot for this book is coming together in a fun way, with lots of twists and turns and exciting sequences. I have lots more revisions to do, and am looking forward to the back-and-forth with my editor that always improves my books so much. It's hard work, but enjoyable as well.
So there you go! That's what I'm working on now. Thanks for asking, fictional person I created for the purpose of this post.