Hands down, the best commentary I have ever read (not just on Ecclesiastes but on all Biblical books). Highly technical book that is thoroughly researched. Attention to linguistic detail will push your Hebrew to the next level; comparison to several ancient near eastern texts adds highly textured details to Qohelet; analyses of larger literary context illuminates several passage; conversations with several scholars leads to some really good insights. After reading Seow's commentary, other commentaries (not just on Ecclesiastes) feel so insufficient. If you can get only one commentary on Ecclesiastes, this is the most comprehensive one!