Koi2Kichi - skip amazon etc and go straight to source eg :- Peter Waddington
Infiltration - Exclusively Koi - Home and order off him in the UK - unfortunatly you guys are paying the price for the strong pound at the moment which is nearly $2 to the pound.
Alternativly check out E-bay / koi sites for 2nd hand versions - i picked up my copies this way and only paid $120 for both books inc p + p.
These books are a must have IMHO even if they end up a just reference material.