The transition from the end of Raihousha-hen to Book 12 of the source material is unfortunately not particularly seamless.
There’s some notable overlap between events, namely Minami’s and the Saegusa and Yotsuba twins’ introductions. The skyscraper opening ceremony there meanwhile is actually a near-total alteration of a welcome-home party in the book. An understandable change, seeing as how the original version introduced a bunch of new characters (some of whom are never seen again) and they probably wanted to end the series with a bang.
What really sticks out though is the density of the text, which shouldn’t have been a surprise considering how wordy the Anime series is. It takes some time to get used to the conversational style, which frequently skips between similarly named characters and occasionally has a rather odd syntax. I wonder if they used multiple editors? When/if you acclimate yourself to the oddities though the general experience is not unlike watching the show, featuring a similar mix of dialog, action, and comedy.
At least for the 12th through 15th books.
The 16th however, which (perhaps not incidentally) is the shortest of this bunch, throws a couple pretty severe curve balls which seem completely out of character. Frankly, the way the page count had been steadily decreasing since the 12th book already had me on edge… but then to go and devote what little space they had to such highly questionable developments on top of that has me feeling that there’s no need to continue following the franchise.