This is possible with iOS 6 or later.  Apple have introduced a way of locking access to a single app via a PIN code.  

This feature is known as 'Guided Access'

To set this up,  

1) Go into the iPad settings app and choose General->Accessibility->Guided Access

2) In the Guided Access area turn it on and enter a passcode/PIN

Guided Access is then set up.

3) To use this with Foliobook,  open Foliobook,  then click three times quickly to enter Guided Access. Note that Guided Access is capable of more than simply locking an iPad to a single app, but most will not need that extra capability.  If you do need that, then google 'Guided Access iOS 6' for more online information. 

(Foliobook can be set to read-only mode if an iPad is to be distributed to clients, or left with someone, this prevents editing the portfolio.  Use F->Portfolio->Lock to lock Foliobook.  Note that this can only be unlocked by going into the settings app and selecting Foliobook and unlocking it there, so once you have turned this on,  if Guided Access is also turned on, its not possible to re-enable portfolio editing without first cancelling guided access.)