Michael Davidson maintains an excellent family of websites, many sponsored by manufacturers, which are an excellent source of microscopy information:
Fundamentals of Light Microscopy and Electronic Imaging, Douglas B. Murphy: A good general purpose introduction to commonly used microscopy techniques, including fluorescence, phase contrast and DIC, and brief discussions of confocal microscopy and cameras.
Handbook of Biological Confocal Microscopy, 3rd ed., James Pawley, ed.: This is the book to get if you want an in-depth understanding of not just confocal microscopy, but the whole diversity of modern microscopy techniques. It's not a good introduction to the subject but it is a fantastic reference work with lots and lots of technical detail.