套书

kě yǐ shuō shì hé gè gè nián líng duàn de hái zi de shū dōu yǒu ér qiě duō shì yǐ tào shū de xíng shì chū xiàn

可以说适合各个年龄段的孩子的书都有，而且多是以‘套书’的形式出现。

It can be said that there are books for children of all ages, and they are mostly in the form of 'sets'.