I'm afraid this is a fairly grim list. There is one historical event that took place mainly on what is Polish soil, and which has inspired a vast amount of searing literature. That was itself in the middle of a global conflict in the course of which Poland was first partitioned and then reassembled with its borders shifted substantially to the west. So it's not very surprising that the top book by ownership on the list of books tagged "Poland" on LibraryThing and Goodreads is a memoir/novel largely set in Auschwitz and written by a survivor, though it starts in what is now Romania and ends at Buchenwald which is in Germany. Published in 1958. it is:
Night / La Nuit, adapted from the original longer Un di Velt Hot Geshvign, by Elie Wiesel.
The second book is also set at Auschwitz, but is very much a work of fiction (indeed it has been criticised for a number of implausibilities compared with the awful historical reality), and a lot of the action takes place in Berlin. By an Irish writer (which makes me somewhat uncomfortable - it wasn't really our Holocaust to write about), published in 2006 with a 2008 film adaptation, it is:
The Boy in the Striped Pajamas, by John Boyne.
However, that only makes a clear second place if you count separately the different versions of the best-known graphic novel about the Holocaust, originally published in two volumes (published in 1986 and 1991) but these days more widely available as a single book (published in 2003). The combined ownership of the first volume and the combined book on LibraryThing exceeds the Irish author's by some way, though it falls short on Goodreads. Its framing narrative is set in 1970s New York, but the main story is in Poland before and during the Second World War, with a small section in Germany. It is, of course:
Maus, by Art Spiegelman
These are the most widely owned books set in Poland; but they are not as frequently tagged "Poland" by LT and GR users as a non-fiction memoir by a Warsaw woman, who together with her husband was able to conceal and ultimately rescue several hundred Jews by imaginative utilisation of his place of work. It is:
The Zookeeper's Wife, by Diane Ackerman (based on the unpublished diary of Antonina Żabińska)
Bubbling under, The History of Love, by Nicole Krauss; Schindler's List / Schindler's Ark, by Thomas Kenneally; Sophie's Choice, by William Styron; and many more. Edited to add: Also in this range, The Tin Drum, though a lot of it is set in post-war West Germany.