So, turns out that illustrating a book is a crazy big job, and this one doesn't even have that many pages! (about 17 illustrations in the end). I've hardly been on dA for weeks, so I'm sorry for all the late replies to your lovely comments!
The author I'm working with is wonderful, she's been really great about letting me have a lot of creative control over how the pictures end up, and gives good feedback. She's brand new to this though so it's interesting working with her coz there are some things that I totally take for granted talking to you wonderful dA people which I forget others don't know - for example that you can't expect someone who works with traditional media to suddenly switch to digital for one project and be just as proficient with it! (Luckily the author is ok with me sticking with my watercolours!)
I'm learning so much from this project, for example things about composition, fitting multiple people into scenes, drawing faces with different emotions (my sister pointed out yesterday that in the past practically none of the people in my paintings are ever smiling - oops!) and drawing people of different ages. The only problem is that as soon as I've finished a picture for the book, I can think of ten ways to make it better, but I don't have time with the tight deadline to go and redo them all ten times! Oh well, I guess everyone is continually improving, otherwise what would be the fun of it?!
Here are a couple of details from some of the not-so-rough sketches for the book (I'm doing rough sketches, not-so-rough-sketches, rough colours, lineart, and final paintings as separate stages each of which gets checked off by the author before I continue).