下列句子有何分別？

get at = reach 手觸到
get = obtain; take 拿到
I cannot get at the book on the shelf.=
我的手未能觸及架上那本書。
I cannot get the book on the shelf. =
我拿不到架上那本書。

I cannot get at the book on the shelf.= not quite right, better rewrite it as

I cannot reach the book on the shelf
or I cannot get to the book on the shelf.

我的手未能觸及架上那本書。

I cannot get the book on the shelf. = OK
我拿不到架上那本書。