I recently purchased all 3 "best of White Dwarf" Compilations. FotR and TTT are terrific, with lots of great articles and battle reports (where they really shine, IMO). RotK, on the other hand, has no battle reports but instead a lot of rules for figures (many of which can be found in the One Rulebook) and is about half the length of the first two. I highly recommend the first 2, but can't say that the RotK one lives up to their standard.

I can't buy them anywhere.
Did you buy it in a GW? Or in a small retailer?

I would guess he got them off e-bay. I see them semi-regularly there.

You would be correct. I got the first two as part of a double-offer and the third one separately.