You may want to look at the research by Warwick University
"The use of real books is recommended for three reasons.
The first is that children's long-term success in reading depends on the way they adapt to texts, which in turn, depends on the material they read. Children need to encounter a wide and diverse range of books which reflect the literary structures they will experience as their reading progresses. This will help to ensure that they can apply their skills to books varying in style and difficulty. -Where- -vocabulary- -is- -heavily- -controlled- -within- a -reading- -scheme-, -children- may appear to progress but may ultimately have difficulties in transferring their skills to books which are not part of the scheme.
The second reason is that children are more likely to learn the distinctive and critical features of individual words when they are encountered in many different contexts. For example, if children only see the word 'and' written in one font, size, colour etc., they may not appreciate its defining features as readily as seeing the same sequence of letters in numerous fonts, sizes and colours in a variety of books.
The third reason is that the use of reading schemes can only be justified if their structure is different from and easier than that of non-reading scheme hooks. We have explored this within the ERR through looking at four types of texts; a random selection of children's literature (e.g. The Tiger that Came to Tea, Not Now Bernard) and adult literature (fiction and non-fiction) was compared to two reading schemes, Oxford Reading Tree and Rhyme World. The first thing we looked at was the frequency of the 100 most commonly occurring words in written English. Their representation across the four sets of materials was remarkably similar and accounted for approximately 50% of all the words in all four text types. Furthermore the first 16 most commonly occurring words accounted for approximately 25% of all the words. So these high frequency words occurred as often in real books as they did in the carefully structured reading schemes. As a result, the key question is whether there is any value in teaching any additional sight vocabulary, as well as the 100 most frequent words? Our research shows that the return on teaching the 100 common words from the National Literacy Strategy list is high, around 50% of all words in adult and children's texts, but the return on teaching any additional common words, as recommended by the NIS at key stage 1, is very low, less than 1% (after excluding phonically regular words).
Which is why I would suggest to the parents of the children that I teach to leave the reading schemes to school/home reading time and if they want to buy books for their child to buy one of the many high quality picture books widely available.