The ZLB in My Favorite Textbook
As far as I know, among basic textbooks only Krugman/Wells even talks about the liquidity trap.
This is probably a true statement. It is not that other books don't cover the topic, however. It is just that Paul Krugman doesn't know it.
FYI, here is what the leading introductory text says about the topic:
Other economists are skeptical about the relevance of liquidity traps and believe that a central bank continues to have tools to expand the economy, even after its interest rate target hits its lower bound of zero. One possibility is that the central bank could raise inflation expectations by committing itself to future monetary expansion. Even if nominal interest rates cannot fall any further, higher expected inflation can lower real interest rates by making them negative, which would stimulate investment spending.
A second possibility is that the central bank could conduct expansionary open-market operations with a larger variety of financial instruments than it normally uses. For example, it could buy mortgages and corporate debt and thereby lower the interest rates on these kinds of loans. The Federal Reserve actively pursued this last option during the downturn of 2008 and 2009.
Some economists have suggested that the possibility of hitting the zero lower bound for interest rates justifies setting the target rate of inflation well above zero. Under zero inflation, the real interest rate, like the nominal interest, can never fall below zero. But if the normal rate of inflation is, say, 4 percent, then the central bank can easily push the real interest rate to negative 4 percent by lowering the nominal interest rate toward zero. Thus, moderate inflation gives monetary policymakers more room to stimulate the economy when needed, reducing the risk of hitting up against the zero lower bound and having the economy fall into a liquidity trap.