which,that
which(that )I found on my way home 是定语从句,修饰the book 的.
what不是关系代词,它不能引导定语从句.
但如果去掉The book 后,就可以了.
what I found on my way home 是名词从句在句中做主语.
此句中的what还可以看成复合关系代词：what=the book which(that).
所以又有：what I found on my way home =the book which(that) I found on my way home .
大伙看看,上面说的是不是挺有意思的.