Source: NCSS Statistical Software
How to access: It is on the software vendor site and accessible for free. No license for the software is required to access.
Contents: They are basically training documents for using the PASS sample size calculation software. However, each document contains detailed derivations for the sample size calculation, which would be useful to understand how to figure out the sample size. References in each document are also helpful.
Example(s) used in this: Examples are not specific for HIV/AIDS research but more general.
Prior math/stat knowledge to fully understand materials: The documents include mathematical formula. Understanding of the concept of hypothesis testing would be necessary.