Her books are like a comfort food menu. You don't need to eat the dishes in a specific order; you just need to know which ones are part of the same 'family style' meal.
The Slains trilogy is a family-style meal—meant to be shared and enjoyed together. The rest are individual, delicious plates. So, order the family meal if you're in for a long, satisfying sit. Or just order the plate that sounds good tonight. Either way, you'll be full and happy. No need to overthink the sequence of courses.