Be aware! With most words you can just add -er to it, but:
If the word ends in e, you only add -r (dancer, not danceer)
If the spelling is: consonant, vowel, consonant, you double the last letter. (Swimmer, not swimer) (= wrong in book!)
Except when it's a 'y' at the end. You don't double the y (play = player, not playyer)