I bought my repair book speculatively. Subsequently, as repairs became necessary I googled to get a feel for what the normal costs for that repair were and the cost for the parts. Then I looked at my book to see how much sense the instructions made to me and decided from there whether I thought I could get the job done for less time than the price difference would be worth, roughly speaking. I didn't learn anything new to make the estimate and my only previous automotive repair experience was having changed my oil once or twice with my Dad as a teen. I did not think of myself as mechanically inclined at all, but had been working with a group of mechanical engineers for a few years which gave me the confidence that if I broke something while learning, I could buy a replacement part or, worst case, pay an expert to fix my error.