Zur deutschen Version dieses Artikels 
…what does one expect there? Medium to very hard, right? After all, many books have four or five difficulty levels. Well, the small “Su-Doku plus” book (Nr.6, German, from french publisher Éditions Megastar) advertises “Level 3 to 5” on its cover (and contains also two puzzles of level “5-6” 
) – but apparently, they are using a scale up to 10 or so (no, nothing was specified there), for as far as I can tell after solving several of various levels, “3” = “very easy” (you can write it down in a few minutes – “1” must be size 6×6, then…) and “5” = “medium”, at most a few “medium to hard” (8 to 15 minutes, never needed to write alternatives with a pencil – including those that should be “extra difficult” as “2 sabres” should indicate).
Of course, the more practice you have, the faster you become – but the difference between the supposedly advertised difficulty and your expectations shouldn’t be that high…
Hope I’ll find something better among that plethora of offers (not buying that many, anyway)… at least this one had a handfull of other number puzzles, too…