It's Not Rocket Science, summarized below
Bradley is often asked if it's a good time to invest. The answer is more complicated than the questioner would like. Rather than timing the market, investors should always seek to invest towards a well thought-out asset mix that makes sense for their situation.
This asset mix need not be rigid, however. It can be flexible enough such that if one particular market is overvalued (e.g. domestic equities), the mix is shifted in favour of other assets. However, it's important to realize that nobody knows what markets are going to be hot in the future (if they did, prices in those markets would already have been bid up). As such, too flexible a policy could result in a portfolio that can be great, or can go bust. Hindsight bias makes us think what happened in the markets was obvious, when it actually wasn't at the time.
Bradley also notes some research suggesting that investor portfolios are weighted more strongly towards recent positive price performers, at the expense of poor price performers. This is a recipe for poor prospective returns.