I have just returned from the Somme - this book is 'the Bible', an absolute must for visitng the region (you'll find most visitors and B&Bs have them!). It is well laid out and easy to follow, and covers everything you want to know. I would advise having a brief read before visting the area, as it is worth planning your trips in advance. I also like the way it gives a brief story behind the memorials (or important people mentioned). I have one small criticism, there are some errors in the text or the map, but you can sort them out between the text, the map and the pictures in the book!
I found the map very useful (it was the only map of the area I had with me) and from it I found everything I wanted to!!