Oh, figuring out the order for this one is a bit of a puzzle because it depends on how you define the series. I've seen a lot of debate in reader groups. The core is definitely the three main novels: 'The Alpha's Princess', 'The Alpha's Princess: Royal Blood', and 'The Alpha's Princess: Crown of Thorns'. You need to read those in that order or the character progression makes zero sense.
However, there are also a few side stories or companion novellas floating around, like 'The Alpha's Princess: The Beta's Oath' and 'The Alpha's Princess: Luna's Secret'. Most people slot those in after the first book, but they're not essential to the main plot—more like bonus content for side characters. Some publishing sites list them all jumbled together, which is super annoying. My advice is stick to the trilogy first, then explore the extras if you're still hooked.
Honestly, I tried reading 'The Beta's Oath' first by mistake and was totally lost on who half the characters were. The emotional beats in 'Crown of Thorns' hit way harder if you've followed the main couple's journey from the very beginning. I'd save the side stories for later.