Libraries are a great resource for this, believe it or not. If your library uses the Libby app, search for Lisa Regan. The series will often be grouped together, and Libby usually lists series entries in the correct numbered order. It's not always publication order, but for a straightforward series like this, it's the same. Plus, you can borrow them!
Can we take a second to appreciate how prolific she is? To have a list long enough to cause confusion is a testament to her output. We're lucky to have so many books to sort through!
Most of the suggestions here are great. My personal vote is for the Fantastic Fiction website. It's clean, ad-free-ish, and just gives you the facts: title, year, series note, and cover art. It's my bibliographic bible for any author.
You know, I tried the audiobooks for this series, and the narrator absolutely nails Josie's gritty determination. Starting with 'Vanishing Girls' on audio throws you right into the atmosphere—the small-town tension, the rain-soaked scenes, it's all so vivid.
It's a different experience from reading, but equally compelling. Just a thought for anyone who consumes books that way. The performance adds another layer to Josie's character right from the first chapter.
I approached it like a TV series on Netflix. I looked up the order, added all the titles to my 'Want to Read' shelf on Goodreads in that sequence, and then treated it like a playlist. I’d finish one, mark it as 'read', and the next one was right there waiting. Goodreads’ series feature is clunky, but using a custom shelf works. It gamified the process. Watching the 'read' count go up was satisfying. The order was never in question because I’d pre-loaded it. Highly recommend this method for any long series.
Plot-wise, you won't be lost reading out of order. But theme-wise, you might miss a subtle progression. Her early books often focus on a woman in immediate, personal danger from a specific threat. Her later work expands to look at systemic family secrets, the dangers of technology, and the fragility of community. Seeing that expansion happen in real-time, book by book, is a study in how a writer's interests evolve with the culture.