Hi, Reallytired. This is a long post, but hope it is helpful. I used some of the later stages of the RM books when DS2 was learning to read. Each book focuses on a particular sound and its different spelling correspondences, so they can seem to vary in difficulty. They get progressively harder througout each colour set.
For info, here are some brief extracts from the top 3 levels:
Yellow Level Book 1: The duckchick
Mama hen had a problem. Things began to go wrong in the spring, when she was sitting on her nest on her six eggs. Egg 6 was very big. Mama hen sat on it a lot and went "Cluck" at it a lot, but she felt a bit upset that it was so big.
Blue Level Book 1: Barker
Let's start at the beginning. With Grandad's slippers. Barker had lots of fun with them. One night, when Grandad was at a darts match, Barker got hold of one of the slippers and ripped it apart! Grandad said "No, Barker". And Barker just laughed.
Grey Level Book 1: Big Dude to the rescue
This is Luke. The big bloke with the shaved head and the tattoos. And this little pug with the blue velvet collar is Luke's dog. His name is Bruce. Cute, isn't he? Not so much little as minute! Luke likes to take Bruce to the park. Bruce chases sticks (well, twigs) and barks at the ducks, while Luke sniffs the roses or reads 'Gardening Today', humming a little tune.
I would say that the yellow books would probably be a good match for ORT 5 or so. The sentences tend to be longer, but decodable. By the grey books, the sentences are quite long and some of them are more enjoyable for a 5 year old than others. DS was in early Reception when he read them.
I bought the black and white books which are good value and thought they were excellent - comprehension questions and speed words to read fluently, which DS bizarrely enjoyed.
Let me know if you want any more info. You used to be able to download samples from the website, but not since ORT bought them....