Speaker design question-DIY/techies please help

What do the abbreviations Q, Qtc and SQL mean in subwoofer design? I built my first DIY subwoofer using a software program to design the box and I'm happy with the results, but I don't know what some of the terms mean.

Also, what does the following mean:

Low Q Sealed 0.55 ft³ sealed (Qtc=0.6)
Mid Q Sealed 0.35 ft³ sealed (Qtc=0.7)
High Q Sealed o.25 ft³ sealed (Qtc=0.8)
Small SQL 0.5 ft³ tuned to 35 Hz, Ø2" x 11.5"L

In the above, I know that they are refering to the size of the box and to the port diameter and length in the last line. I just don't know what low Q, Mid Q, high Q, Qtc and small SQL mean.

Also, please recommend a good book on speaker design. My primary interest is subwoofer design.

Thanks,