Thunderbolts is a series of leveled reading books covering popular non-fiction topics, with particular appeal to older readers. Each book contains a non-fiction section followed by a fiction story on the same topic. There are four sets in the series. Each set contains six books and is leveled to allow progression. Set Three increases the amount of words on each page, offering progression from the books in Set Two. The texts focus on high frequency vocabulary, with all words being phonically decodable wherever possible. Reading Books Set 3 includes all six reading books.