You could look at used book stores or thrift stores for a hard copy/reference for an more recent version like MS DOS 6.22 too. I still have mine and actually referred to it a few months ago. I am glad I know DOS commands although I have forgotten more than I remember now. In 1999 some very esoteric commands were covered in my A+ exam. Even with Windows 7, understanding how it works with commands like diskpart comes in handy. There is DR DOS and also free is FreeDOS that supports 32 bit protected mode. One last tip is to help remember syntax or switches is to type /? at the end of a command. Open a prompt and type for example chkdsk /?
Edit: Just noticed Maguscreed posted using the help switch too.