Chemical handbooks are the experimental chemist's most important resource. They contain practical guidelines on chemical methods or techniques and various properties of chemical compounds.
Common types of chemical handbooks:
Reaction and Synthesis Guides describe preparations of individual compounds, applications of particular reagents, or general reaction/synthesis methods. The content is grouped by type of reaction, type of starting material, or type of product. These are most useful if you're trying to find out how a certain material is made or how a particular reaction works.
Analytical Methods Guides deal with specific or general analytical chemistry techniques, grouped by analyte, matrix, or method.
Comprehensive Works are usually an ongoing series attempting to summarize all of a given area of chemistry. Good examples include the Beilstein Handbook of Organic Chemistry, and the Gmelin Handbook of Inorganic Chemistry.
Other chemical handbooks provide spectral information on compounds (e.g. infrared spectra, nuclear magnetic resonance spectra, etc.). Examples are the Aldrich Index of FT-IR Spectra and the Aldrich Index of FT-NMR Spectra. These would come in useful if you had spectral data from your own research and were trying to identify a compound you've synthesized.