A popular way to narrow down a particularly broad area of interest is to focus on a specific time period. If you're a medical professional and find books about surgeries fascinating you might find it less daunting to begin collecting texts strictly from the Victorian era, for example.
Doing this also has the benefit of giving your collection a specific feel as it is often the case that books from a specific decade or era will often have a particular style of binding or artwork which was prevalent at the time. One need only glance at the dust jacket art of novels to from the 1920s next to those from the 1960s to see the juxtaposition, this also works with older books as specific binding styles became en vogue at certain points in history.
Use Advanced Search to find books by time period.