docbook.rb: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/share/sgml/docbook/xsl-ns-stylesheets-1.75.2/epub/bin/lib/docbook.rb - from package: docbook5-style-xsl
/usr/share/sgml/docbook/xsl-stylesheets-1.75.2/epub/bin/lib/docbook.rb - from package: docbook-style-xsl

You may install the required package with command yum install {package}

Error in other languages:
docbook.rb: bevel nie gevind nie
docbook.rb: comandă negăsită
docbook.rb: comando não encontrado
docbook.rb: commande introuvable
docbook.rb: command not found
docbook.rb: käsku ei ole
docbook.rb: không tìm thấy lệnh
docbook.rb: komanda nerasta
docbook.rb: Kommando nicht gefunden.
docbook.rb: kommandot finns inte
docbook.rb: komut yok
docbook.rb: nie znaleziono polecenia
docbook.rb: níor aimsíodh an t-ordú
docbook.rb: no se encontró la orden
docbook.rb: no s'ha trobat l'ordre
docbook.rb: opdracht niet gevonden
docbook.rb: parancs nem található
docbook.rb: perintah tidak ditemukan
docbook.rb: príkaz nenájdený
docbook.rb: příkaz nenalezen
docbook.rb: команда не найдена
docbook.rb: командата не е открита
docbook.rb: 命令找不到
docbook.rb: コマンドが見つかりません