Depends on how much you are prepared to spend.

Have a look at the CSIRO Publishing web page http://www.publish.csiro.au/nid/18/bcid/48.htm

For Eucalyptus in the southern half of Australia it would be hard to go past the Euclid CD. For Acacia likewise the Wattle CD has a wealth of descriptive information about ALL the Australian spp.

The 2 Acacia and 1 Eucalyptus volumes of Flora of Australia are comprehensive though the later is now somewhat outdated. Neither it nor the Euclid CD recognises Corymbia as a distinct genus - in the case of the CD because its principal author is strongly opposed to its recognition (in the face of majority botanical opinion in Australia), in the case of the Flora because it predates publication of Corymbia.

For a more general book at a more reasonable price it's hard to beat the current edition of Australian Native Plants by Wrigley and Fagg.