This means that you now get...
...a full colour booklet depicting each level of the tower,
...a similar (ink saving) black & white version of the booklet,
...and ten image files that can be used in conjunction with your favourite VTT.
So, if you've already purchased the booklet, you've probably just received an e-mail letting you know that the product has been updated.
But if you've not purchased it yet (and if Helroth's Tower looks like something you might be interested in), then HERE is a link that'll allow you to pick it up at a discounted price (which lasts until the end of the month).
(*note that these maps are scaled at 50 pixels per grid square).