The following resources contain additional information about d-orbital splitting:
1. This reference goes into more depth with high spin vs. low spin complexes, as well as strong vs. weak field ligands from the spectrochemical series.
Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley: Hoboken, NJ, 2005; pp 10-13.
2. This reference discusses strong and weak field ligands with respect to the pairing energy of electrons in the d-orbital.
Shriver; Atkins. Inorganic Chemistry, 4th ed.; Oxford University Press, 2005; pp 462-463.
3. This reference is a great resource for understanding how pi donor and pi acceptor ligands affect the split in d-orbital degeneracy, from a MO perspective.
Miessler, G. L.; Tarr, D. A.; Inorganic Chemistry, 4th ed.; Prentice Hall, pp 377-381.