Binney and Merrifield, 4.1
van den Bergh, S. 1998, Galaxy morphology and classification (Cambridge: Cambridge University Press). A short, authoritative, and modern review of galaxy classification.
Hubble, E. 1936, The realm of the nebulae. The classic description of galaxy morphology
Sandage A. , and Bedke, J. 1994, The Carnegie Atlas of Galaxies. The definitive description of Hubble's classification scheme and a beautiful collection of galaxy photographs. Should be on every astronomer's coffee table.
Sandage, A. 1975, in Galaxies and the Universe, ed. Sandage, A., Sandage, M., and Kristian, J. (Chicago: University
of Chicago Press), 1. A very good review of the principal photographically based classification schemes, and
especially of their early history.
de Vaucouleurs, G. 1959, Handbuch der Physik 53, 275. de Vaucouleurs' refinement and revision of the Hubble
Elmegreen, D. M., & Elmegreen, B. G. 1987, ApJ 314, 3 - classification based on spiral arm appearance
Nakamura, O., et al. 2003, AJ 125, 1682 - visual Hubble classification of 1500 SDSS galaxies
Ball, N., et al. 2004, MNRAS 348, 1038 - Classification of SDSS galaxies using neural nets
Web-based image collections:
Anglo-Australian Observatory images by David Malin - possibly the best available galaxy images, but limited mostly to the southern hemisphere
NOAO image gallery - 150 galaxy images
Astronomy picture of the day - roughly 1000 high-quality images on various astronomical subjects
SDSS images on Robert Lupton's web page
NYU image gallery - over 400 images of nearby galaxies from SDSS
or just use Google