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
2 Chronicles 17:1
ch2 17:1JEHOSHAPHAT REIGNS WELL AND PROSPERS. (Ch2 17:1-6)
Jehoshaphat . . . strengthened himself against Israel--The temper and proceedings of the kings of Israel rendered it necessary for him to prepare vigorous measures of defense on the northern frontier of his kingdom. These consisted in filling all the fortresses with their full complement of troops and establishing military stations in various parts of the country, as well as in the cities of Mount Ephraim, which belonged to Jehoshaphat (Ch2 15:8).
2 Chronicles 17:3
ch2 17:3he walked in the first ways of his father David--He imitated the piety of his great ancestor in the early part of his reign before he made those unhappy lapses which dishonored his character.
and sought not unto Baalim--a term used for idols generally in contradistinction to the Lord God of his father.
2 Chronicles 17:4
ch2 17:4and not after the doings of Israel--He observed with scrupulous fidelity, and employed his royal influence to support the divine institutions as enacted by Moses, abhorring that spurious and unlawful calf-worship that now formed the established religion in Israel. Being thus far removed, alike from gross idolatry and Israelitish apostasy, and adhering zealously to the requirements of the divine law, the blessing of God rested on his government. Ruling in the fear of God, and for the good of his subjects, "the Lord established the kingdom in his hand."
2 Chronicles 17:5
ch2 17:5all Judah brought . . . presents--This was customary with the people generally at the beginning of a reign (Sa1 10:27), and with the nobles and high functionaries yearly afterwards. They were given in the form of voluntary offerings, to avoid the odious idea of a tax or tribute.
2 Chronicles 17:6
ch2 17:6his heart was lifted up in the ways of the Lord--Full of faith and piety, he possessed zeal and courage to undertake the reformation of manners, to suppress all the works and objects of idolatry (see on Ch2 20:33), and he held out public encouragement to the pure worship of God.
2 Chronicles 17:7
ch2 17:7HE SENDS LEVITES TO TEACH IN JUDAH. (Ch2 17:7-11)
Also in the third year of his reign he sent to his princes, . . . to teach in the cities of Judah--The ordinary work of teaching devolved on the priests. But extraordinary commissioners were appointed, probably to ascertain whether the work had been done or neglected. This deputation of five princes, assisted by two priests and nine Levites, was to make a circuit of the towns in Judah. It is the first practical measure we read of as being adopted by any of the kings for the religious instruction of the people. Time and unbroken opportunities were afforded for carrying out fully this excellent plan of home education, for the kingdom enjoyed internal tranquillity as well as freedom for foreign wars. It is conformable to the pious style of the sacred historian to trace this profound peace to the "fear of the Lord having fallen on all kingdoms of the lands that were round about Judah."
2 Chronicles 17:9
ch2 17:9the book of the law--that is, either the whole Pentateuch or only the book of Deuteronomy, which contains an abridgment of it.
2 Chronicles 17:11
ch2 17:11Also some of the Philistines brought Jehoshaphat presents, and tribute silver--either they had been his tributaries, or they were desirous of securing his valuable friendship, and now made a voluntary offer of tribute. Perhaps they were the Philistines who had submitted to the yoke of David (Sa2 8:1; Psa 60:8).
the Arabians--the nomad tribes on the south of the Dead Sea, who, seeking the protection of Jehoshaphat after his conquest of Edom, paid their tribute in the way most suitable to their pastoral habits--the present of so many head of cattle.
2 Chronicles 17:14
ch2 17:14HIS GREATNESS, CAPTAINS, AND ARMIES. (Ch2 17:12-19)
these are the numbers--The warriors were arranged in the army according to their fathers houses. The army of Jehoshaphat, commanded by five great generals and consisting of five unequal divisions, comprised one million one hundred and sixty thousand men, without including those who garrisoned the fortresses. No monarch, since the time of Solomon, equalled Jehoshaphat in the extent of his revenue, in the strength of his fortifications, and in the number of his troops.