For spectroscopy, I'd go to the online handbook put out by Richardson Gratings:

http://gratings.newport.com/information/handbook/handbook.as...

For a textbook, this one is pretty good:

http://www.amazon.com/Introduction-Spectroscopy-Donald-L-Pav...

For optics, Hecht is the classic text:

http://www.amazon.com/Optics-4th-Edition-Eugene-Hecht/dp/080...

I'd suggest Keating though, since it is a more comprehensive overview and includes things like the human visual system and some basic math review:

http://www.amazon.com/Geometric-Physical-Visual-Optics-2e/dp...

For instrument design in general, these books are outstanding:

http://www.amazon.com/Building-Scientific-Apparatus-John-Moo...

http://www.amazon.com/Optical-System-Design-Second-Fischer/d...

Of course, any understanding of spectroscopy should include some background of the physics behind what you're measuring. Any modern physics book would be fine: Krane, etc.:

http://www.amazon.com/Modern-Physics-Kenneth-S-Krane/dp/1118...

Haven't tried this one, but it looks cool:

http://www.amazon.com/Spectroscopy-Reading-Stellar-Practical...