Wikipedia's take on Design of Experiments, and in particular, see Fisher's principles.

OpenIntro notes: Sections 1.4-1.5.

An example of poor experimental design. The published manuscript and the post mortem.

Be careful when analyzing publicly available data. The published manuscript and the post mortem.

RR Sokal, FJ Rohlf (2011). Biometry, 4th edition, WH Freeman. This text is required. Everything else is not madatory, only listed as suggestion in case you want to do some additional reading and/or exercises.