Binding on new books

I just bought a brand new book and within 50 pages of reading it the part of the book by the spine where the pages connect to the cover has separated from the spine. Now when the book is open you can shove a pencil clear under the pages and the spine. IS this normal for a book, have books always done this. Usually I buy used books and they frequently do this, but I thought it was unusual that a new book started off connected to the spine and quickly came unglued, if it is normal, why do they glue it down in the first place?