Molecular Basis of Biological Clocks

While, being published in 1999, it is not the most up-to-date paper, this is certainly the most thorough yet most clearly and coherently written review of the molecular basis of circadian rhythms in cyanobacteria (Synechococcus sp.), plants (Arabidopsis thaliana), fungi (Neurospora crassa), insects (Drosophila melanogaster) and mammals (Mus musculus):

Molecular Bases for Circadian Clocks by Jay Dunlapp