百日の説法屁一つ: “a hundred days of sermonizing, one fart” → a metaphor for how extreme effort something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds

平日: weekday; the "say" radical (曰)

(bookstore jargon) stacking books on top of each other, facing upwards

tideland

Short for ひやけサロン.

to sit on the fence; to be opportunistic

平地: flat land

鵯, 白頭鳥: bulbul

皇妃: an empress / 口碑: a legend / 工費: construction costs / 公比: a common ratio; a geometric ratio / 公妃: a duchess / 公費: public expenditure / 叩扉: beating on a door; a visit to someone

叩扉: to beat on a door; to visit someone / 光被: to spread widely (said of a light, or of magnanimous favor of someone in power)