There are a number of good introductory texts on astronomical photometry. For a gentle introduction to the use of CCDs in photometry, try some of the articles in ‘amateur’ astronomy magazines, such as the one by Kaitchuck, Henden and Truax.
A good, detailed introduction is Astronomical Photometry by Henden and Kaitchuck. The similarly titled Astronomical Photometry — A Guide by Sterken and Manfroid is a much more technical and rigorous treatment. Another useful and detailed text is Straižys’ Multicolor Stellar Photometry. There are also a number of good articles in the conference proceedings Astronomical CCD Observing and Reduction Techniques.
The chapter ‘Photoelectric Reductions’ by Hardie in Astronomical Techniques is a classic treatment of the reduction of photometric observations. However, it predates the introduction of CCD detectors and seems a little dated now.