Programming Ruby: The Pragmatic Programmers' Guide by Dave Thomas, Chad 
Fowler, Andy Hunt (ASIN 0974514055) for $44.95
Not yet released.

Ruby Developer's Guide by Michael Neumann, Lyle Johnson, Robert Feldt, 
Lyle Johnson, Jonothon Ortiz (ASIN 1928994644) for $32.97
Usually ships in 11 to 12 days

Sams Teach Yourself Ruby in 21 Days by Mark Slagell (ASIN 0672322528) 
for $28.39
Usually ships in 1 to 3 weeks

Ruby In A Nutshell by Yukihiro Matsumoto (ASIN 0596002149) for $16.47
Usually ships in 24 hours

The Ruby Way by Hal Fulton (ASIN 0672320835) for $27.19
Usually ships in 24 hours

Making Use of Ruby by Suresh  Mahadevan, Suresh Mahadevan, Rashi Gupta, 
Shweta Bhasin, Madhushree Ganguly, Ashok Appu (ASIN 047121972X) for $23.80
Usually ships in 24 hours

Game Programming with Python, Lua, and Ruby by Tom Gutschmidt (ASIN 
1592000770) for $26.39
Usually ships in 24 hours

Figure 1: Output from Listing One.