That's exactly what I got from it, you buy more pages divided in packs and keep adding and re-arranging them at will. The concept seems odd at first, but this is rather genius, as it allows the book to be quite literally "never-ending", as they can simply release new information that you can add to it. With packs like the "Gomu Gomu no Mi Vol. 1" there's also the indication that we'll likely get other pages in the book that include things like devil fruits, locations, ships, etc. This really can turn out to be a never-ending book of knowledge!
–- Update From New Post Merge ---
By the way, not sure if it's been brought up already, but judging by the listings I'm seeing a pattern: The first booster pack in each month is part of the first half of the series, while the second is about the second half (Sabaody onwards). First booster set is late East Blue - Sabaody, second is Arlong Park - Impel Down, third Alabasta - Whitebeard Pirates, fourth Baroque Works - Fishman Island, fifth Enel's forces - New Fishman Pirates. Seems like each of them progresses to cover half of the story, meaning we might get 10-12 other booster packs total after that before it catches up (though they might add more stuff afterwards).
10 - 12 other booster packs, assuming they're sized consistently, should put the total number in the ballpark of the 400ish the folder is built to hold as well. If I've done the math right, everything currently announced + 12 extra boosters would make 384 cards. Exactly enough for one more booster pack to reach 400, actually, which could be made up of promo cards or could just be one more pack.