Dr Robert Brander is a coastal geomorphologist who's got a superhuman knowledge of beaches and rips. So knowledgeable is he that he's taken on a superhero nom de plume - Dr Rip!

You may have seen Dr Rip on the tele dropping purple dye into rips at Bondi and Tamarama giving colour to the water movement. He's also written a book on the subject: Dr Rip's Essential Beach Book is the final word on how beaches are created and what forces - such as rips - impact them. Written in a friendly informal style, it's the book Dr Karl would've written if he surfed.

For the next seven days the eBook version of Dr Rip's Essential Beach Book is on sale at iTunes. Usually $11.99 it's just $4.99 until January 17th.

You can get it at Dr Rip's website or at iTunes.