What Andreas Gierit said but I will expand a little as you are missing a variable
You need a number of the book you are on which starts at zero and increments each time you press "Y" so it starts like
int onBook = 0;
You created an array of books called b now you use it (why create it if you dont use it) .. this line is wrong
scanf("%s %d\n",book.bname,&book.bid);
should be
scanf("%s %d\n",b[onBook].bname,&b[onBook].bid);
Get it onBook is the book index in array b your are loading the data into.
I will leave it to you to increment onBook when you press "Y" and you probably need to make sure it doesn't increment pas the length of the array b.