There is no "normal" PSA reading. For example someone could have PC with a PSA under 3, or someone might not have it with a reading considerably higher.
The trend in PSA readings is more relevant than one individual number, and most look for any sudden trend rise defined as three higher peaks.
PSA Density: PSA readings are meaningless without factoring in prostate size. The prostate naturally produces PSA, and the larger it is the more PSA is produced: "PSA Density" is one simple calculation of the ratio of PSA to prostate size which by some criteria should be less than .15.
A "free PSA" test will indicate the % of PSA being produced by normal means and provide a probability of PC. It's often used when the PSA is in the gray area between 4 and 10.
Very large proportion of PSA readings in the range of 4 to 10 are tracked down to benign causes.