When it comes to determining where to specialize in the intellectual life, Sertillanges doesn't offer any special advice because his aim is different. But, since determining my special focus was not straightforward, I thought I would offer a few thoughts that could help you if you're trying to determine yours.
If you're reading this post, I would imagine that you're in one of two situations. Either you're deciding in general what specialty to select - Law, Economics, English, whatever. Or, you're trying to decide within a particular specialty. There are many ways to approach this complicated decision and my advice here is not meant to be comprehensive, but rather a contribution to the chorus of voices available on this point.
Let your history of professional and academic interests guide you.
This may seem like an obvious point, but I think that it is often overlooked or at least under-appreciated when the wheels of anxiety are turning. Perhaps my story can be illustrative.
I was preparing for a phone conversation with a friend of mine at the University of Dallas precisely about The Intellectual Life and I was hoping that he would indulge my penchant for fantasy by talking with me about all the interesting fields of expertise I had considered over the past few weeks.
"I love Pride and Prejiduce. Maybe I should study Literature. Or, perhaps write the great American Novel."
Instead, he gently guided me towards the more obvious solution - economics and in particular finance. Of course, if you saw my CV, you would say, Duh!, but I wanted to overlook that temporarily in favor of getting my imagination all worked up about what could be.
Now, there's quite a bit of common and spiritual sense in letting your history of professional and academic interests guide you. God puts us where we are for a very specific reason and his providence aligns events perfectly for the mission that he has for us. And, thankfully, he has taken all your and my foibles and indecisiveness into account! Take a look at where God has led you professionally, academically or otherwise and follow this path to its logical conclusion.
"But," you might respond, "the intellectual life is supposed to be a professional change, a break from my regular work. Why would I want to dig into the same field in greater depth?"
For starters, your ability to get started in a field that you're somewhat familiar with is going to be much easier than the alternative. You'll know more people in that area and have an idea of who the major thinkers are. That aversion that you have to pursuing the "logical choice" may be based on your dissatisfaction with your field, which is understandable. This dissatisfaction may be a sign, not that you need to abandon the field altogether, but rather that you might be called to make a contribution towards improving your field.
Of course, these sorts of reflections can only take you so far, but they hopefully will eliminate a lot of the imaginings that distracted me from accepting my area of focus.