I have a test with terms like "Performance IQ" and "Visual-Spatial", I would like to understand them better.
Can somebody recommend a book?

Google (http://www.google.com)is your friend.

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Performance IQ is often compared to Verbal IQ.

It there is a BIG DIFFERENCE (PIQ greatly exceeds VIQ or PIQ greatly exceeds VIQ), it *MAY* indicate an LD ('learning disability') diagnosis.

I'd GOOGLE

Neuropsychology Assessment
or
Psychoeducational Assessment

I'll be glad to get you some info at work

Here's a link to a fairly long and comprehensive article. It may be a bit dense, but does answer some of the questions you were asking. Part III is the part that's most relevant,especially the part about subtest scatter.

http://www.nldontheweb.org/wright.htm

If it was easy to Google, I woudn't ask:-) I would like to really understand the terms, like understanding exactly what kind of problem I would be good or bad at.