If you fail a paper three consecutive times, a six-month waiting period applies before you can rewrite. If you'd like, I can:
Widely available in academic PDF formats. It provides excellent visual breakdowns of the Rankine cycle, gas cycles, and entropy—core components of the 2nd Class A3 exam.
Complex calculations for efficiency, emissions control, and boiler performance.
Standard texts by organizations like ASHRAE offer excellent diagrams and formulas for the B2 refrigeration syllabus.