This is Dyslexia has been very well designed and is targeted at children with bags of information, a brilliant video that describes what life is like for dyslexics in comic form and suggestions for parents and teachers. Huge bonus is the fact that it is UK made so the voices have English accents (loved the Geordie character in particular!).
iOverlayPlus is a very simple app but effective , it utilises the device's camera and puts a coloured overlay over the words in your book, in exactly the same way that plastic overlay sheets do but I think using an iphone would look much cooler! The £1.99 price tag is a little steep but it is very easy to use.
ClaroSpeak UK isn't cheap at £3.99 either but it is worth the money in my opinion. You can type directly into the app, paste in text from elsewhere or open a file from your Dropbox and the app will then read it back to you. There is a choice of 4 voices , 2 English, 1 French and 1 Spanish and you can change many things in the settings including the speaking rate and text colour etc. The ability to save the texts as audio files and email them to yourself is very useful. Yes the text to speech is a little disjointed but it's pretty good.
Of course many of the built in idevice facilities are very useful too, in particular VoiceOver (Settings/General/Accessibility/Vision) but it's a shame this will work in iBooks but not the kindle app, worth experimenting with even so.