What methods exist for reading out content to hear correct pronunciation? I would like to be able to have selected content from ebooks and webpages read out to me so that I may hear the correct pronunciation of words and sentences.

What accessibility options exist to help me do this using existing apps like Safari and Kindle or iBooks, and do 3rd party apps exist that are specifically written to narrate selected text or contents?
Several built in tools exist to assist with this sort of thing, but many of them are intended to help people with limited vision etc, whereas I understand that you want to hear particular words etc possibly as a learning tool.  You can still configure certain settings to help you do this.

In Settings &gt; General &gt; Accessibility you can use Voiceover to read out all screen interaction, allowing you to also perform a quick 3 finger down swipe across text to have it narrated to you.  There are various options for altering the lnaguages and dialects, the voice pitch and quality etc, but it does all tend to compromise using your device for anything else unless you are of limited vision etc.

In the same area, there is also Speak Selection; this is far more subtle, allowing you to select any text via the usual cut/copy/paste techniques, to which the option to Speak the selection is added, and you can configure various options as per the screenshot below.



This latter method is ideal for speaking snippets of text because it is a system tool, and will allow for the technique to work in any app in which you can select text.  The downside is that it is intended to be an accessibility tool for visually impaired people, and the pronunciation of the text is still very much "computer speak", resulting in some words being pronounced oddly or even incorrectly as all the narration is being synthesized  rather than using a base of real natural language to form words.



In the same text menu, you also have the Define, allowing you to pull up a dictionary definition of the word.  I've never been able to make any sense of the pronunciation hints in dictionaries that appear next to names, but if you can, then here it is to see (You can also change dictionaries if appropriate, you should be prompted to download one if you don't already have one)



As for 3rd party apps, I don't know of any that are available which have the specific purpose of reading aloud content with a primary purpose of pronunciation, this sort of tihng often requires large and complex databases containing snippets of genuine voice recordings which can be cut together to read aloud text with a far more natural sound that many of the simpler synthesized techniques as per above.