It looks like this might be selfpubbed, but I'm not entirely sure.
In any event, Hill describes a doctor's visit in his late teens when a doctor refers to his adoptive status, assuming he knows. Hill doesn't say anything to his parents at the time, heads off to college, then marriage, kids, etc. Along the way, when his dad's health deteriorates, dad comes clean and reveals there's a (half?) brother and urges him to track his brother down. The book is about Hill's mission creep. He starts out trying to find the brother (and identify his deceased mother) and succeeds relatively promptly. His efforts to find his father are a lot more complex and take several more decades and significant developments in DNA. Fairly early on, he gets hold of his original birth certificate and not too much later a full copy of his file with the probate court, but his mother misrepresented who the father was (early DNA testing confirms who dad isn't) thus creating some problems.
I'm a little stunned at how effective Hill was at reconstructing the erratic dating life of a very young woman so many decades earlier. Despite his efforts, however, real progress does not occur until he gets access to online matching services for Y-DNA tests. That gives him a surname and in conjunction with his notes, point him in (approximately) the right direction.
Hill's a big proponent of DNA testing for adoptees; his own tale is an great read and a solid argument for that strategy. But even if you have no particular use for the information for yourself, it is still a heckuva story.