Usage of recommend?

“I can't recommend the book enough” is somewhat ambiguous. It might mean the speaker likes the book tremendously, beyond his or her ability to say how much. Or it might mean the speaker cannot recommend the book.

A similar sentence, “I can’t recommend him highly enough”, is referred to at literalminded.wordpress.com as a “carefully ambiguous phrasing”.

The following, in which most sentences exhibit similar ambiguities, is a classic example of a recommendation that may look positive but is indeed quite negative. It is quoted from a Deliberate Ambiguities webpage at www.cut-the-knot.org.

You write to ask me for my opinion of X, who has applied for a position in your department. I cannot recommend him too highly nor say enough good things about him. There is no other student of mine with whom I can adequately compare him. His thesis is the sort of work you don't expect to see nowadays and in it he has clearly demonstrated his complete capabilities. The amount of material he knows will surprise you. You will indeed be fortunate if you can get him to work for you.