ebbv: You're right that you can't learn everything about legislation by reading it in isolation.
You generally need to read it side-by-side with the law it's amending. You need to read any amendments, and the rejected amendments. You need to read caselaw on point so you're aware of how courts have interpreted certain terms. You need to read existing laws in the same area. To the extent that courts will consider it, you need to read the legislative history, floor debate, and committee reports. Ideally you'd want to talk to a lawyer specializing in this area. Obviously the shorter the law, the less all this is necessary, but the text of the law tends to be merely a starting point.
You generally need to read it side-by-side with the law it's amending. You need to read any amendments, and the rejected amendments. You need to read caselaw on point so you're aware of how courts have interpreted certain terms. You need to read existing laws in the same area. To the extent that courts will consider it, you need to read the legislative history, floor debate, and committee reports. Ideally you'd want to talk to a lawyer specializing in this area. Obviously the shorter the law, the less all this is necessary, but the text of the law tends to be merely a starting point.