I'll go check
http://www.sjgames.com/in-nomine/ -- that links all the books currently published.
Core Rules are stock #: 3324 (blue hardcover printing) and 3301 (softcover flaming feather). There's also the black and white hardcovers; I don't know if those had different stock numbers from the softcover, or from each other.
Game Master's Guide is 3317.
Angelic Player's Guide is 3307
Infernal Player's Guide is 3309
Corporeal Player's Guide is 3315
Ethereal Player's Guide is 3318
Liber Canticorum is 3312
Liber Castellorum is 3313
Liber Reliquarum is 3310
Liber Servitorum is 3314
Liber Umbrarum (e23) is SJG82-1121
You Are Here is 3316
Superiors 1 is 3320
Superiors 2 is 3321
Superiors 3 is 3322
Superiors 4 is 3323
Lilith (e23) is SJG82-1101
Eli (e23) is SJG82-1102
The GM pack (Feast of Blades, GM screen) was 3302
In Nomine Anime is 3325
The Calendar was 9011
Night Music is 3303
The Marches is 3305
Heaven and Hell is 3306
Fall of the Malakim is 3308
The Final Trumpet is 3311
I can only speculate that they kept a few numbers "free" for projects that were not yet envisioned, or that never got off the ground. Either that, or there was a typo and no one cared enough...
Alternatively, there might have been GURPS books which slipped in somehow, and rather than change a GURPS book number at a late date, they might have just decided to live with it. But I'm not going to go checking the GURPS books to see!