I had these books as a child and have bought them now because I was a little unimpressed with the modern reading books.
The modern reading books seem to have no science behind them at all. For example, a modern book for a 5 year old will throw in the word 'invisible' or 'skeleton' because it suits the story. It certainly doesn't suit the 5 year old trying to read it. And they will frequently use hard to read fonts, italics, words like 'yeah', 'Kaboooooom!!!!!' and litter the pages with exclamation marks. You can tell that they have no educational basis behind them at all.
The peter and jane books were designed by a teacher with experience of teaching the 'hard to teach'. Simple repetition gives the child a feeling of success. The specially designed font is the easiest font for young readers to see (and has become a classic). The illustrations are simple and not distracting. And the key thing, the books get incrementally more advanced. They never throw in unreadable words for the sake of the story. Instead they bend the story to suit the words.
You simply can't get better books for teaching your children to read. Ignore all the comments about them being old fashioned. If making reading easier for kids is old fashioned, then call me old fashioned.