Complex sentence: The boom was reflected by the extension of credit to a dangerous degree, including in the stock market, which rose to record high levels, which in retrospect after the Stock Market Crash of 1929 were dangerously inflated.
Split into simpler sentences: The boom was reflected by the extension of credit to a dangerous degree, including in the stock market, which rose to record high levels. It became apparent in retrospect after the Stock Market Crash of 1929 that credit levels had become dangerously inflated.