I'm pretty sure this is the intended reading order for the continuity: 1. Switched, 2. Fangtastic!, 3. Re-Vamped!, 4. Vampalicious!, 5. Hollywood Secrets, 6. Love Bites, 7. Eternal Flame. Don't start with any of the 'Star Style' spinoffs first, even though they might be tempting—they assume you know the characters from the later main books. The flow makes more sense this way.
The definitive answer is five core books. The sequence goes: 'Switched' (where the human and vampire sisters meet), 'Fangtastic!' (dealing with school and fame), 'Re-Vamped!' (summer camp adventures), 'Vampalicious!' (a fashion-themed mystery), and 'Fang Me!' (the big finale). This order tracks their growing bond and the unraveling of the secret behind their separation.
Any list showing more than five is likely including the subsequent 'Star Style' series or special edition compilations. The main storyline is contained and concluded within those first five volumes, which is quite satisfying.
It's not just about plot; it's about fashion evolution, too! Olivia's style and Ivy's goth look are described in detail, and they slowly start to borrow elements from each other as the series goes on. Reading in order lets you track that visual character development, which is a fun, subtle detail. In 'Switched', their styles are completely separate symbols of their worlds.
By 'Immortal Hearts', there's a noticeable blending, reflecting their fused lives. You miss that narrative-through-costume if you read haphazardly. Sometimes the simplest series have these lovely little through-lines.
All this order talk is making my head spin. Can't we just agree they're all good books and let people read them however they want? Gatekeeping the 'correct' order feels anti-fun.
If you're reading to a younger sibling or your kid, definitely go in order. The excitement of 'what happens next' is a huge part of the charm. The chapters often end on little cliffhangers that propel you right into the next book. Keeping the sequence intact preserves that magic.