Just to clear up one thing, by the way: the なぜ？どうして？ book we read previously was なぜ？どうして？せきのふしぎ 二年生. It’s different from, though related to, the book we’re currently reading, which is なぜ？どうして？科学のお話 二年生. Note that they’re both second-year books. The other sorta-active book in the ABBC is another in the same set, 10分で読める物語 二年生 - also second-year.
We elected to skip the first-year books because they almost completely lack kanji, which in some ways makes them even more difficult to read. It’s the first two stages of learning kanji - 1. Why are there so many kanji? 2. Why are there so FEW kanji in this book?