Do not be tense. Go through past paper you will have an idea about the type of questions asked. You need not to go through whole book. Details like diseases of crops, insects etc can be skipped. Try to cover the chapter which is about soil science. A question or two will be asked from that. Paper comes on general issues. Like last time environmental pollution was a question. Read the economic survey of Pakistan, chapter on agriculture. Book by Akhtar Abbas is a good one to have. Rest of topics can be covered from internet.