The Scientist and Engineer's Guide to Digital Signal Processing (excellent free book)
dspguide.com
dspguide.com
require 'open-uri';(1..34).to_a.each{|x| open("CH#{x}.PDF", "wb").write(open('http://www.dspguide.com/CH1.PDF').read)}
PS: Those who are wondering wtf this is all about, save it as book.rb and run 'ruby book.rb' and it will grab all the PDFs of that book for you.
require 'open-uri'; (1..34).to_a.each{|x| open('http://www.dspguide.com/CH'+x.to_s+'.PDF'){|u| File.open("CH#{x}.PDF","wb"){|f|f.puts u.readlines}}}