买means "buy". 我打算买一本书./I am going to buy a book.
买到means you have already got sth.
我买到那本书了!I‘ve got that book! / I've bought that book!
我已经买了那本书. I‘ve (already) bought that book.
when you use"买到",it seems like the thing you bought is not so eazy to get.
太好了,我终于买到那本书了!Couldn't be better,I got the book at last !
买到那本书了吗?Have you got that book?