I have the Bob Carss book, but rate the Scott-Donelan one as better. Could just be personal preference. The other thing, at the time I got the Carss book a couple of years back, they were out of print and it was a little bit of a hunt to find a good one second hand.
I know that just seeing horse and people tracks could be a bit boring, but equally, up at Ashridge, there are so many deer that trying to track just one over the leaf litter is a pain. I got quite a kick at Merthyr Mawr from tracking people for as far as I gould. The hard part was finding an area where my pathetic lack of ability in time stamping a track wasn't a total liability