Note: it's been a hot second since I read it, so much of this review is off memory.
I never read books out of order, but with Riverbound, it was completely by accident. Even still, once I got past the beginning and all the side characters (but let's face it, I'm used to dense adult epic fantasies, so honestly, it just reminded me of their huge casts, and I had zero problems with it, hahaha), I didn't feel like I missed anything. (Though I do plan on reading Heartseeker!)
The events of book 1 were well-introduced from the beginning, and Only's life was established clearly, so in my "I thought this was book 1" lens, I felt like Heartseeker's events were an interesting background to the plot and Only's motivations.
While this is a Middle Grade fantasy, it is very enjoyable for adults to read as well! The writing and story was well-crafted, the characters were fascinating, and there was great political intrigue seen through the eyes of Only. I liked the steward and the princess quite a bit, and I also liked the relationship she had with her father the king. I kind of liked the king, actually. I felt bad for him.
The villain was pretty clear from the beginning, and that was probably the intent--the biggest mystery was moreso, "Is she actually telling the truth, or did she find a way to evade Only's skill?" At times it killed me to read Only trusting her, but it made sense. She relied so much on her skills to discern lies that she wasn't listening to her heart and the people around her.
The climax!! The resolution was so satisfying, and funny too! Behold, the quote that has stuck with me since I finished reading it last year:
"Not only that, Highness," Lark explained to Hauk, "but she planned to weasel her way onto the throne of Thorvald by wedding her daughter to your brother!"
One of Orrad's eyebrows shot up so far, it nearly launched itself off his forehead. He gave Adalise a lewd smile, earning a slap to the back of the head from his infuriated father. (pg. 248)
There was great LIFE throughout this--a bustling setting filled with celebrations and worldbuilding and different lands and cultures and characters. I adored the side characters as much as the main characters. I loved the humor and lightheartedness but also the seriousness and the strong convictions and goals Only had.
If you want an entertaining, rich MG fantasy, pick this (duology) up! If you want a lighter, heartwarming read with intrigue and depth similar to fantasy geared toward an older audience, pick this (duology) up! You're perfectly free to start on this book, but you should probably read them both...in order. ;)