ebook-viewer: 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 Debian possible default full paths are:

/usr/bin/ebook-viewer - from package: calibre

You may install the required package with command apt-get install {package}

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