If you think you are a beginner, I recommend LAMMPS tutorial as below :
1.https://lammps.sandia.gov/tutorials.html (direct way for programmers)
2.MIT course atomistic computer simulations (to learn basics)
3.A Practical Introduction to the Simulation of Molecular Systems by Martin J. Field (Best book to start)
for professional researcher :
I recommend to use LAMMPS manual that you could find in installed directory.
for Developers :
Do What you like under LAMMPS License as in
GNU Public License.