• Shabbat 30b records that the Sages wanted to suppress Ecclesiastes because its words contradicted each other, until they found that its beginning and end were words of Torah — Kohelet's opening "vanity of vanities, all is vanity" was not suppressed because the Talmud understood it as strategic intelligence: the entire apparatus the Sitra Achra uses to entice the soul — wealth, pleasure, power, legacy — is denominated in vapor, and the Kohelet's declaration is a tactical intelligence briefing on the enemy's balance sheet.
• Megillah 7a debates whether Ecclesiastes "defiles the hands" (i.e., is canonical holy scripture) and concludes it does because "its beginning is Torah and its end is Torah" — Proverbs 1's opening is wisdom, but Ecclesiastes 1's opening is horror: "vanity of vanities" is the Tzaddik who has surveyed the Sitra Achra's entire world and submitted his field report — Solomon's greatness made him the one man who could audit the enemy's holdings and declare them illiquid.
• Berakhot 57b teaches that Ecclesiastes is among the five books a person should read if he dreams of a Torah scroll — Ecclesiastes 1:4 "one generation passes away, and another comes, but the earth abides forever" is the Talmudic time-perspective correction for the warrior who becomes demoralized by the short horizon of earthly campaigns: the Sitra Achra is a temporary force deployed within the permanent earth, not the other way around.
• Avot 4:17 ("Better is one hour of repentance and good deeds in this world than all the life of the World to Come; and better is one hour of spiritual bliss in the World to Come than all the life of this world") cross-illuminates Ecclesiastes 1's vanity declaration: the warrior who accepts Kohelet's verdict is freed from the Sitra Achra's leverage entirely — if all of this world is vapor, then the Sitra Achra's bargaining chips are worthless.
• Sanhedrin 91b uses Ecclesiastes in the debate on resurrection and afterlife — Ecclesiastes 1:18 "in much wisdom is much grief, and he who increases knowledge increases sorrow" is the Talmudic cost-of-intelligence doctrine: the Tzaddik who fully understands the scope of the Sitra Achra's empire carries a grief-burden proportional to his wisdom, but this grief-as-wisdom is the specific quality that qualifies him for the deepest levels of the counter-operation.