A good resource on allocation and it's impact on risk and return would be "The Ivy Portfolio". Available from Amazon as Kindle ebook from Australia. If you don't want to buy the book there's a PDF paper on asset allocation by the author which can be down loaded for free. Not quite a good, but a good primer. You could use this strategy if you wanted a buy and hold ETF portfolio or it goes into market timing if you are happy to be a bit more hands on and disciplined. I use a similar strategy in our investment portfolios, with a bit of discretionary trading on the side to keep my interest.
Another good resource for asset allocation using ETF's would be Paul Merriman. He has his own website which has a lot of free stuff, plus he has quite sensible podcasts free through iTunes. I've not read his books, so can't comment on those.
In Oz, we are starting to get a good range of ETF's from iShares, Vanguard and others. Some of them are just secondary listings of the US based ETF, so there's a bit of paper work for US authorities, but it's relatively easy. We've got (or have had recently) an ASX 200 fund, a small all ords fund, an S&P 500 fund, a global 100 fund, a Europe fund - haven't had metals, ag or bonds for a long while. I don't include real estate in our portfolios as we have direct property investments.
I've read HEAPS of investment books over the years, but these 2 resources are what I refer my friends to if they ask the right questions. Whether you do the buy & hold or the timing method, depends on your personality. I like to be fairly hands on, plus I could not, under any circumstances, stomach a 50% fall in my portfolio. Add to that the fact that these big falls in your portfolio early into your retirement markedly decrease the chances of your money lasting as long as you do.