It's a collection of sketches he drew on location, some impressions of the people from the past, and other assorted sketches he overlay onto the photos taken. There's a lot on Spanish architecture and much on the history but everything is written in Japanese.
This book is actually a limited edition. A 108-page hardcover that comes with a DVD (region code 2) and several calendar cards feature the art in the book.
There's a pepita facebook page that you can also check out.
This is the book trailer.
Below are some videos from the DVD.
「"pepita" 井上雄彦 meets ガウディ」DVD chapter 5 "ガウディの生家"
「"pepita" 井上雄彦 meets ガウディ」DVD chapter 7 "職人魂"
「"pepita" 井上雄彦 meets ガウディ」DVD chapter 14 "元模型職人"
If you buy from the links, I get a little commission that helps me get more books to feature.
Here's the Amazon Japan buying guide for your reference.