If your memory is as mine, good but short, then you might find PocketChris, a set of iOS photo apps, very useful. 'PocketChris' doesn't say much so there is a pretty good chance that you might overlooked them.

The series includes 3 separate apps and the best thing is that they are for free. The creator, Chris Marquardt, a German photographer and the host of TFTTF podcast, put together most of the information that you'll need to refresh your memory on composing, exposure, metering, sensor sizes, etc. - basically most of the things you need to know about digital photography. All these information is divided into 3 apps, from basics to advanced.

I think it's nice to have all these information at the palm of your hand and you can refresh your knowledge about digital photography when waiting for a bus or, if you're a man, when you're waiting for your wife/girlfriend trying on a 10th dress in her favourite clothing store :)