I've tried solving this to no avail so far. I think I've followed the instructions in your book correctly to this point.
DEBUG [64c2c32d] Running /usr/bin/env [ -d /usr/local/rbenv/versions/2.1.5 ] as deploy@(example).com
DEBUG [64c2c32d] Command: [ -d /usr/local/rbenv/versions/2.1.5 ]
DEBUG [64c2c32d] Finished in 1.087 seconds with exit status 0 (successful).
INFO [fb0a4d08] Running /usr/bin/env sudo /etc/init.d/unicorn_ffth_a2_production restart as deploy@(example).com
DEBUG [fb0a4d08] Command: ( RBENV_ROOT=/usr/local/rbenv RBENV_VERSION=2.1.5 /usr/bin/env sudo /etc/init.d/unicorn_ffth_a2_production restart )
DEBUG [fb0a4d08] reloaded OK
INFO [fb0a4d08] Finished in 0.032 seconds with exit status 0 (successful).
INFO [131af9e6] Running /usr/bin/env sudo /etc/init.d/unicorn_ffth_a2_production restart as deploy@(example).com
DEBUG [131af9e6] Command: ( RBENV_ROOT=/usr/local/rbenv RBENV_VERSION=2.1.5 /usr/bin/env sudo /etc/init.d/unicorn_ffth_a2_production restart )
DEBUG [131af9e6] Couldn't reload, starting 'cd /home/deploy/apps/ffth_a2_production/current; bundle exec unicorn -D -c /home/deploy/apps/ffth_a2_production/shared/config/unicorn.rb -E production' instead
DEBUG [131af9e6] master failed to start, check stderr log for details
cap aborted!
SSHKit::Runner::ExecuteError: Exception while executing as deploy@(example).com: sudo exit status: 1
sudo stdout: Nothing written
sudo stderr: Couldn't reload, starting 'cd /home/deploy/apps/ffth_a2_production/current; bundle exec unicorn -D -c /home/deploy/apps/ffth_a2_production/shared/config/unicorn.rb -E production' instead
master failed to start, check stderr log for details
...
I, [2015-02-03T04:05:41.420675 #3975] INFO -- : executing ["/home/deploy/apps/ffth_a2_production/shared/bundle/ruby/2.1.0/bin/unicorn", "-D", "-c", "/home/deploy/apps/ffth_a2_production/shared/config/unicorn.rb", "-E", "production", {10=>#<Kgio::UNIXServer:/tmp/unicorn.ffth_a2_production.sock>}] (in /home/deploy/apps/ffth_a2_production/releases/20150203081053)
I, [2015-02-03T04:05:42.056269 #3975] INFO -- : inherited addr=/tmp/unicorn.ffth_a2_production.sock fd=10
I, [2015-02-03T04:05:42.059709 #3975] INFO -- : Refreshing Gem list
/home/deploy/apps/ffth_a2_production/shared/bundle/ruby/2.1.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:206:in `pid=': Already running on PID:3975 (or pid=/home/deploy/apps/ffth_a2_production/current/tmp/pids/unicorn.pid is stale) (ArgumentError)
from /home/deploy/apps/ffth_a2_production/shared/bundle/ruby/2.1.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:135:in `start'
from /home/deploy/apps/ffth_a2_production/shared/bundle/ruby/2.1.0/gems/unicorn-4.8.3/bin/unicorn:126:in `<top (required)>'
from /home/deploy/apps/ffth_a2_production/shared/bundle/ruby/2.1.0/bin/unicorn:23:in `load'
from /home/deploy/apps/ffth_a2_production/shared/bundle/ruby/2.1.0/bin/unicorn:23:in `<main>'
We've got an old pid and server pid is not the old pid
killing master process (good thing tm)
We've got an old pid and server pid is not the old pid
killing master process (good thing tm)
We've got an old pid and server pid is not the old pid
killing master process (good thing tm)
I, [2015-02-03T04:05:44.192262 #3975] INFO -- : master process ready
I, [2015-02-03T04:05:44.206371 #4067] INFO -- : worker=1 ready
I, [2015-02-03T04:05:44.208465 #4063] INFO -- : worker=0 ready
I, [2015-02-03T04:05:44.221720 #4071] INFO -- : worker=2 ready
I, [2015-02-03T04:05:44.452826 #3740] INFO -- : reaped #<Process::Status: pid 3752 exit 0> worker=2
I, [2015-02-03T04:05:44.453221 #3740] INFO -- : reaped #<Process::Status: pid 3760 exit 0> worker=0
I, [2015-02-03T04:05:44.553920 #3740] INFO -- : reaped #<Process::Status: pid 3748 exit 0> worker=1
I, [2015-02-03T04:05:44.554506 #3740] INFO -- : master complete
I've tried restarting nginx and rebooting Ubuntu, didn't work.
Any advice would be greatly appreciated. Thanks!