Since the first goal of book collecting should be to collect books that you are interested in it shouldn't be much of a surprise that collections will often be built around a specific kind of fiction or non-fiction topic. Even if you do not consider yourself a "book collector" you can probably take a look at your bookshelves and find the beginnings of this kind of collection simply based on your reading tastes.
There is no limit to how broad or specific your collection can be, and how you go about choosing the books you collect is entirely up to you. However if you are worried about overwhelming yourself there are always ways to narrow down different genres or fields of interest to something more manageable.
For instance if you are particularly interested in photography or art you might start with your favorite movement or art scene such as Art Deco or Pop Art. Another possible angle would be to document the history of a specific type of book illustration technique like lithography or wood blocks. You may also be interested in the history of art and would collect books about the artists, such as biographies, rather than books about the art.
Similarly if you were keen on literature and fiction, this is the largest category in publishing and would need to be broken down into mystery books or science fiction. Even if that is too general you could dissect science fiction into cyberpunk, steampunk, the golden age, alternate histories, space opera and more. If you prefer keeping your collection slightly more general, and even want some new reading suggestions built in a good route might be books which have won a particular award such as the Pulitzer Prize or the National Book Awards.
If you prefer real life happenings to make believe there are hundreds of choices when it comes to collecting non-fiction. Again you could focus in on a combination of interest and time period such as sport in the Victorian era or tank manuals from World War II. You could zero in even further on a specific event such as the Declaration of Independence or the 1919 Black Sox scandal. Or simply choose a kind of book from a specific era. Examples of this could be books about astronomy from the Renaissance which were printed on vellum or Georgian era medical books still containing their original boards. The world is your oyster here and you should really peruse whatever makes you happiest.