I am sure this is becoz, at times your Request.QueryString("bookId") doesn't carry a value as I dont see the other 2 reasons fit in your case, as there is no other cause for this error, othere than these 3.
* Not enough parameters
* Misspelt column names
* Incorrect delimiters with SELECT queries

What datatype is the ISBN? Also check if you have mispelt anything there.

Try printing your sql statement every time on the page, and try to reproduce this error and see what the sql statement looks like for the case that result in this error.

Check out this - No value given for one or more required parameters.

Hope that helps.
Cheers!

_________________________
- Vijay G
Strive for Perfection