A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
1 Kings (1 Samuel) 29:1
sa1 29:1DAVID MARCHING WITH THE PHILISTINES TO FIGHT WITH ISRAEL. (Sa1 29:1-5)
Aphek-- (Jos 12:8), in the tribe of Issachar, and in the plain of Esdraelon. A person who compares the Bible account of Saul's last battle with the Philistines, with the region around Gilboa, has the same sort of evidence that the account relates what is true, that a person would have that such a battle as Waterloo really took place. Gilboa, Jezreel, Shunem, En-dor, are all found, still bearing the same names. They lie within sight of each other. Aphek is the only one of the cluster not yet identified. Jezreel on the northern slope of Gilboa, and at the distance of twenty minutes to the east, is a large fountain, and a smaller one still nearer; just the position which a chieftain would select, both on account of its elevation and the supply of water needed for his troops [HACKETT, Scripture Illustrated].
1 Kings (1 Samuel) 29:2
sa1 29:2David and his men passed on in the rereward with Achish--as the commander of the lifeguards of Achish, who was general of this invading army of the Philistines.
1 Kings (1 Samuel) 29:3
sa1 29:3these days, or these years--He had now been with the Philistines a full year and four months (Sa1 27:7), and also some years before. It has been thought that David kept up a private correspondence with this Philistine prince, either on account of his native generosity, or in the anticipation that an asylum in his territories would sooner or later be needed.
1 Kings (1 Samuel) 29:4
sa1 29:4the princes of the Philistines were wroth with him--It must be considered a happy circumstance in the overruling providence of God to rescue David out of the dangerous dilemma in which he was now placed. But David is not free from censure in his professions to Achish (Sa1 29:8), to do what he probably had not the smallest purpose of doing--of fighting with Achish against his enemies. It is just an instance of the unhappy consequences into which a false step--a departure from the straight course of duty--will betray everyone who commits it.
1 Kings (1 Samuel) 29:9
sa1 29:9notwithstanding the princes of the Philistines have said--The Philistine government had constitutional checks--or at least the king was not an absolute sovereign; but his authority was limited--his proceedings liable to be controlled by "the powerful barons of that rude and early period--much as the kings of Europe in the Middle Ages were by the proud and lawless aristocracy which surrounded them" [CHALMERS].