John Barton 'Joel and Obadiah' (The Old Testament Library, WJK Press 2001, pp 168 Pounds 25.00 ISBN 0-664-21966-700). This slim commentary on Joel and Obadiah is most helpful, adequately covering both Prophets.
Despite its length, Joel has always been the 'problem child' of Old Testament exegesis. Containing no overt indication of date, it was written sometime between 9th and 2nd Century BCE, with scholars arguing for any date within the Axial period.
Barton argues that it is the work of one author, though later editors may have "improved" the text in some places. An early Second Temple dating is the most probable with no mention of kings, but instead a prominence of the priesthood and an evident division between a theocratic and eschatological party - "most readers are likely to gain an impression of a rather small community gathered round the Temple… somewhere in the 400s". Joel is therefore an important source for the elusive period of Israel's history - the aftermath of the Return from Exile, the disappearance of Zerubbabel and rebuilding of the Temple.
Barton covers topics such as Joel as a prophet (Schriftprophet), the large number of quotations in Joel from other Prophetic Writings like Amos, Isaiah and Ezekiel, as well as providing a detailed analysis of theological themes such as eschatology, the mercy of God, the outpouring of the spirit, and the judgement on the nations. In addition to a 36 page Introduction, he provides a very readable commentary on the text with his own translation.
Perhaps the apogee of Barton's commentary is that for Obadiah - a book whose difficulty, as Jerome noted, is in inverse proportion to its length. Though only 21 verses long, it summarises great prophetic themes, (like the Day of YHWH) with a 'politically incorrect' attitude to foreigners seen as an instrument of chastisement. Barton regards the idea of a 'Deutero-Obadiah (vv.15a & 16-21) as plausible, but suggest that we read each part holistically, and not as pastiche, fitting well into a midexilic period.