While the Gray & Corbally tome is an excellent reference ... in fact the ultimate one ... it is expensive and maybe overkill for most amateurs ... while I have the G&C book I usually go to Gray's "A Digital Spectral Classification Atlas" first as it generally has all the info I need. A pretty good synopsis of the G&C book as zero cost. Download at
http://www.aesesas.com/mediapool/142...latlasGray.pdf