Although I also thought these were called the N series because the first one was written by Douglas Niles.
The idea behind the N series was to provide yet another set of adventures for starting players. It makes sense really. You will have far more low-level characters than high-level ones.
N1 Against the Cult of the Reptile God
N2 The Forest Oracle
N3 Destiny of Kings
N4 Treasure Hunt
N5 Under Illefarn
Of these, it is N1 that interests me the most. It is ranked as 19 of the top 30 greatest adventures of all time and Douglas Niles was an author I have grown to respect over the years; especially for his contributions to AD&D. Plus for me this module was published in that "Golden Age" of adventure design in 1982 when so much great stuff was going on.
Against the Cult of the Reptile God is also the unofficial name I give to my "Second Campaign" adventure series. These adventures include:
N1 Against the Cult of the Reptile God, levels 1-3 (novice)
U1 The Sinister Secret of Saltmarsh, levels 1-3
U2 Danger at Dunwater, levels 1-4
U3 The Final Enemy, levels 3-5
I1 Dwellers of the Forbidden City, levels 4-7
C1 The Hidden Shrine of Tamoachan, levels 5-7
I2 Tomb of the Lizard King, levels 5-7
I3 Pharaoh, levels 5-7
I4 Oasis of the White Palm, levels 6-8
I5 Lost Tomb of Martek, levels 7-9
X4 Master of the Desert Nomads, levels 6-9
X5 Temple of Death, levels 6-10
I9 Day of Al'Akbar, level 8-10 (if I can find a copy)
Gary Gygax's Necropolis, levels 10+
Just so much good stuff out there.
Of the N series, N1 is the best known to me. I have read through it many times and always wanted to do something with it.
I started playing N3 once, back in my college days, but I don't think we got very far.
N4 Treasure Hunt is an interesting one since it is the only AD&D adventure I know of that has the characters start at 0-level. Very interesting choice if you ask me and maybe something I should try for my "Second Campaign".
For people reading this and have no experience in gaming, these are all great places to start. Well except for N2 and nobody likes that adventure!