I can suggest you a little bibliography largely "tested" in educational activities regarding eeg and erp.
As starting points to be introduced to EEG/ERP studies:
Ernst Niedermeyere (2005) - Electroencephalography, 5th Edition.
Kamel Nidal & Aamir Saeed Malik (2014) - EEG/ERP Analysis - Methods and Applications.
Sacid Sanei & J. A. Chambers (2007) - EEG Signal Processing.
Regarding Neurofeedback and more advanced readings::
Budzynski T.H., Budzynski H.K., Evans J.R., Abarbanel A. (2008) - Introduction to Quantitative EEG and Neurofeedback - Advanced Theory and Applications.
Fengyu Cong (2015) - Advanced Signal Processing on Brain Event-Related Potentials - Filtering ERPs in Time, Frequency and Space Domains Sequentially and Simultaneously.
This is not a "rule".
A good placement allows you to have a good probability to fetch something.