A cookbook is only as good as its poorest recipe.

Child - A cookbook is only as good as its poorest recipe - wist.info quote

Julia Child
Julia Child (1912-2004) American chef and writer
Quoted in Frank Prial, “Light’s Still on Julia Child,” New York Times (1997-10-08)
    (Source)

Apparently a phrase she used frequently, as she drew on numerous cookbooks as source material and reference for her own. Another use can be found in an interview with Mike Sager, "What I've Learned: Julia Child," Esquire (1 Jun 2000).

Often given (perhaps from other occurrences) as "A cookbook is only as good as its worst recipe." For example, her obituary by Regina Schrambling, "Julia Child, the French Chef for a Jell-O Nation, Dies at 91," New York Times (13 Aug 2004).