While titled "A Handbook for Beginning Realists", I would not recommend this book for someone who is entirely new to epistemology. I think quite a bit of background knowledge is being assumed regarding figures like Descartes, Kant, Cardinal Mercier, etc... That being said, the book is definitely worth reading as Gilson has plenty of valuable stuff to say regarding philosophical starting points, untethered/abstract vs. tethered/concrete system building (my verbiage not his), and why realists should avoid being roped into arguments undergirded and contextualized by non-realist presuppositions.
It is pretty fascinating that Gilson does not really offer a thorough going refutation of idealism so much as simply an alternative road. The running idea throughout the book can be summarized something along the lines of: you begin your philosophy either with Aristotle or Descartes, but once you make your choice... the conclusion becomes inevitable. As Gilson puts it, if you begin with Aristotle, you end up with Aristotle—as a realist. If you start with Descartes, however, you actually end up with Berkely or Kant—as an idealist. The reason for this is that the realist moves from things to concepts while the non-realist attempts to move from thought to things—the latter being an impossible task. According to Gilson, you are free to pick your starting point whichever way you like, but then you get your just philosophical desserts.
Gilson spends quite a bit of time expounding upon this idea in a few different directions. For one, he spends a significant amount of time critiquing attempts at formulating "critical realism" and explaining why they fail to arrive at realist conclusions with non-realist methods. Gilson also explains the shortcomings of a truly realist approach which seeks to appropriate and redefine Descartes' language in order to unsuccessfully try to bridge the gap in conversation with non-realists. Finally, he critiques the many shortcomings of non-realist starting points and system-building which often lead their adherent to absurd conclusions which run counter to their daily experience.
Having spent most of the book laying a foundation out of all of this background context, it is in the final portion of it that Gilson delivers the actual "handbook" part of the work. In this final chapter, Gilson systematically lays out rules for those who want to embark on the realist path to follow in order to avoid all sorts of idealist potholes. If what you're looking for from this portion is a beginning-to-end explanation of Thomistic philosophy of mind and epistemology, then you will not find that here. What you will find, however, is a lot of insightful commentary regarding the realist vs. idealist debate in epistemology—but one which you need some background knowledge to truly appreciate.
All in all, I think this a great book which I would recommend to someone who: 1. Has already read a contemporary overview introduction to epistemology, and 2. Is already acquainted with, at least, the basic outlines of Thomistic psychology and epistemology.