yes this indeed very easy and very possible.
I am guessing the guest book script you are using was written by someone else.
Go into the code and find out the block of code that actions the guest book entry to be submitted. Once you find this code in php, write your own code using the mail() function, using the variables already available to that part of the script.
...............original code................
/*
sneaky code
*/
//send mail to yourself
mail();
...............original code................
you may want to copy and paste your script so its easier to explain.