Furigana are frequent in many technical and scientific texts, not only in children books, because such texts may include many words that would not be used in normal conversations.
This was already true for the books published before WWII, i.e. before the writing reform, even if those books contained much less hiragana than the modern texts (after the writing reform a lot of hiragana word terminations began to be written in order to disambiguate the readings of kanji for which native Japanese readings are chosen, even when the complete furigana are not provided; this post-WWII writing style may reduce the need for furigana).
This was already true for the books published before WWII, i.e. before the writing reform, even if those books contained much less hiragana than the modern texts (after the writing reform a lot of hiragana word terminations began to be written in order to disambiguate the readings of kanji for which native Japanese readings are chosen, even when the complete furigana are not provided; this post-WWII writing style may reduce the need for furigana).