I (surprisingly) enjoyed books like this. I always find myself falling for the other guy, and I am always screaming when they actually end up together. It's the best feeling in the world. The weird and interesting thing about books like this is you always sort of 'feel' it coming, but at the same time it's unexpected. The author always seems to have a way to foreshadow it and make it so you'll be happy about it in the end. Another reason I liked situations like this is because usually, the second guy had a more hate/love relationship with the heroine than the original romantic interest.
1. Trylle Trilogy by Amanda Hocking
One of the first series of this kind I've ever read. I think Amanda Hocking should receive more recognition for her writings and books. I unexpectedly was very happy with Wendy ending up with Loki instead of Finn, since Wendy and Finn had a hate/love relationship. If you are looking for a series where the girl first falls for the second guy who appeared later on in the series, this is perfect for you. And for those who really enjoyed Rhysand from A Court of Thorns and Roses, you'll find yourself loving Loki, because Loki is really similar to Rhysand. Both are a big arrogant with their 'charms' and is always teasing and flirting with the heroine. However, both heroes truly and deeply love the Protagonist, and has a 'deep side' that they tend to hide.
2. Unearthly Series by Cynthia Hand
The second guy appeared sooner than the first series I suggested. However, I really liked how Tucker and Clara had a hate love relationship started first. And for those who greatly enjoy forbidden love, this is the series for you. Another thing that made this series unique is that for the first time, it's the guy who realized that he had fell for an angel, instead of the other way around. However, this series would not be my first choice if I have to choose, because I think it's a little bit too fast for me. I like it when they don't end up for at least until two books, like the Trylle Trilogy and Young Elites series. The reason though that this series even made it to this list is that although brief, there really was a pretty good hate/love unexpected relationship going on in the first book. If only it lasted longer.
3. Young Elites Series by Marie Lu
If you would die without an HEA then you will hate this series. Because this book had a severe case of bittersweet ending syndrome that made me just want to go ahead and strangle Marie Lu to death. And it's not even the first time she did this - she pulled the same little stunt with the Legend series, and I really, really, really hated her for it. I have to say though, loved Magiano and Adelina here. The thing I loved most about heroine falling for the second guy is that the author somehow always has this amazing ability to make you wish that the guy ends up with the girl, and when they finally kiss, you're just like - YES. This series is more similar to the Trylle trilogy, because Magiano like Loki, did not appear until the second book of the series (which, I emphasize, I love). One thing that I liked about Magiano is that he was reluctant to help Adelina at first, but then later grew to fall in love with her.
4. The Court of Throne and Roses Series by Sarah J. Maas
Although Rhysand appeared in the first book as well, why I really liked it is because Feyre didn't officially 'become a couple' with him until middle/end of the second book. And if you still haven't realized yet, I really don't like it when the relationship escalates too fast. For those who love books where the guy (is a king/prince or in this case, high lord) has to keep up a 'cool, cruel and collected' mask, it's the series for you. And then there's the hate/love relationship.
5. Dragonian Series by Adrienne Woods
This series does not have the exact scenario for when-the-girl-falls-for-the-second-guy, and can be listed because if we're going to go strictly speaking, this series counts. But the main reason that I suggest this series because I really, really liked it. And it's one of the series I am shocked from how much I love it. So basically we already meet the second guy (Blake) in the first book, but actually our protagonist, Elena, decided to date Lucien instead. But then he died in the second book, and we have a severe hate/love relationship in the third and fourth book, Frostbite and Moonbreeze. Either way, you have to give this series some time, because it really is good.