There's nothing wrong with that-- and there's certainly nothing wrong with having a book that is much, much longer.when the authors' writing style is such a pleasure to read, and the two main characters are so well-developed (and, generally speaking, manage to AVOID having stupid mis-perceptions about how they feel about each other, because they actually take the time to think things through).
At the same time, there were some issues. First, there was a serious pacing problem, where a major plot thread got resolved in two sentences after chapters and chapters dealing with the other parts of the plot. It came across as super-super rushed ("The battle took approximately half an hour, and accomplished everything it was supposed to, the end."). Like... there was too much plot to cram into the amount of space that was left in the book, but not enough to be a second book.
My more significant problem (which I am going to discuss in great, spoilerish details after the cut) was something that is... well, it's either that there is a plot hole, or I missed something despite repeatedly looking for it. I'm putting it below the cut; anybody familiar with the book, feel free to chip in.
Okay-- the villainous Spider has kidnapped the heroine's parents, because he's trying to track down a super-weapon (more or less) that her mad scientist maternal grandfather developed. Fair enough. He talks about how "the diary" is key to finding the Evil MacGuffin, and is trying to get the heroine's mother to translate it.
That implies, of course, that he has the diary-- he has somehow gotten the book, but it's in code and he can't read it to find clues to where the MacGuffin is located.
He declares that he's going to have his chief henchman in charge of nasty magical will-sapping modifications work his magic on the heroine's mother-- magic which, by the way, is both disturbing-sounding and really kind of weird and never entirely explained. It is supposed to be similar to what the Spider's own subordinates do to themselves, but accelerated, and it involves fusing the human body with plants (?), and somehow if you do it right, you end up with a plant-human hybrid with no will of its own-- so it will do whatever you say, provided enough of the original cognitive function is left (although if too much cognitive function is left, it will kill you rather than obeying you). That whole part of the plot teetered perilously between "infodump" and "not enough information to make this make sense" with a side of "wait, plant tissue? really?"
While that's going on, at one point, the heroine and her cousin go investigate their grandparents' old house, which has been abandoned for decades and is kind of falling apart into the swamp. Inside, there's a lot of damage (not from the swamp), including fairly extreme damage to the library-- there's a specific statement that her grandfather's diaries are ruined.
Now, I eventually figured out that when the Spider leaves his secret underground lair, he takes the translation with him, and leaves the original diary behind-- so that when Our Heroes retrieve it, they end up with the coded version, and have to figure out how to break the code so that they know what's going on.
What is driving me bonkers (aside from: wait, plant tissue? really??) is... how did Spider get the diary in the first place? I can see how he got on the trail of the mad scientist grandfather. I can see that he clearly had a focus on the grandparents' old house, and made a deal with the sworn enemies of the heroine's family so that he could get access to it while helping them to take it over. That would have been the logical place to get the diary. But, as near as I can figure, the only time any books are mentioned in conjunction with the house, it's in the context of "and the whole house was ravaged and ruined and sacked as if a horde of drunken Vikings on a holiday had rampaged through it, and also the swamp had been frolicking damply throughout for many, many years." In fact, the diaries are specifically referred to as having been completely ruined.
So, where does this one, perfectly-preserved diary come from? The perfectly-preserved diary in which the mad scientist records his Grand Experiment from the first stirrings of the idea to the last crazed rambling scribbles, over years and years? Considering that it trails off into incomprehensible Mad Scientist gibberish (in code), it's not plausible that he would have remembered "put diary safely in box before commencing rampage." And the authors didn't even try to include any kind of "Good heavens, cousin! In the midst of these ruined books, there's a box that looks like it once contained... a book!" hand-waving. Also, what would have marked this particular volume of the diaries as The Important One That Contains Secrets?
*sigh* Doubtless, I am over-thinking this.