The poem is a short, slightly odd glimpse of someone reading, or remembering, a romance. In "her books," it seems, there is always a "strange mechanic," and always a moment when "it is time for sex." The lines sound a little like a bored summary of a formula: the same character shows up, the same scene follows. Calling the mechanic "strange" hints that the speaker finds the pattern both familiar and faintly ridiculous.
Then the tone shifts in the last two lines. The heart is "a little jealous / a little off." The speaker doesn't announce a big feeling. Instead, there is a small, awkward pang, the kind you might not want to admit to. Judging by the wording, the jealousy may be aimed at the fictional lovers, or at whoever "she" is and what she reads. The poem never says which.
The mood is dry, wry, and quietly uneasy. It plays with the word "romance" by setting stock plots beside a real, slightly sour emotion.