I still get a little giddy thinking about this series — it hooked me the way late-night reading sessions used to when I was in high school. If you mean the Lauren Kate 'Fallen' books (the most common one people ask about), the clean reading order is basically publication order, with a companion/novella you can slot in if you want the extra romance beats.
Here’s the straightforward order I follow whenever I recommend it: 'Fallen' → 'Torment' → 'Passion' → 'Fallen in Love' (optional companion novella/short-story collection you can read here) → 'Rapture'. If you want absolutely everything, read 'Fallen in Love' after 'Passion' and before 'Rapture' — it collects character-focused vignettes that fill in emotional gaps but won’t change the main plot. There’s also a later companion called 'Unforgiven' that fans sometimes read after 'Rapture' if they want more world and character closure.
What I loved when rereading was that the main four books carry the primary story arc, while the shorter companion pieces are like dessert — sweet and optional. If you’re starting fresh, give the first two a go; they set up the mythology and the central love story nicely. Oh, and if you’re watching the movie adaptation afterward, read at least through 'Passion' so the ending doesn’t feel too spoiler-y.