Sherwood and the NightWatchMan

So a while back I read this book, Sherwood by Megan Spooner, and it was pretty good (actually, definitely recommend for all Robin Hood fans!), but it really reminded me of how the NightWatchMan came to be. I know we had a little bit of explanation of what brought her to begin with the servant...but the whole costume, where did that come for? I feel like this book could be added to the BBC minus some of the action and Robin's gang (and SPOILER FOR THE BOOK: death).