I want to go to the bookshop that has many books.

在从句里（定语从句that has many books）,that是主语,has是主语的谓语动词.where是一个副词,是不能充当从句的主语的.where只能引导一个地点状语从句,或者出现在特殊疑问句（询问地点的）里面.如,
I like the place where people are very friendly.
(where引导一个从句,表明主语people在那里[where]怎么了)
而上面那句话,that 既引导一个从句,又在句子里充当主要成分--主语.