stuff 1: metal slug x mook.
this is a very good guide for metal slug x only.
stuff 2: neo geo freak magazine.
in neo geo freak magazine you can write nearly all information about metal slug x. If you have a gamest magazine, your informations are complete!
stuff 3: super play video. Super play video is world record by HAMU HAMU from Daytona III game center. This video is in VHS from original neo geo MVS cabinet, this is not OFFICIAL video (not gamest or Scitron video) but Daytona III production.
super play video is sold out from some years, but is possible find it in Japan (in some arcade shop)
for me, exist other stuff about slug x, in other japanese magazine