Painless Spelling (Barron's Painless)

To be quite honest, I couldn't imagine spelling has a lot to do with pronunciation before. The author clearly associates each letter pattern with each vowel sound.(e.g./u; o as in woman, ou as in could, oui as in bouillon, u as in bull, oo as in wood) It sounds like a real cliche, but practice makes perfect; you'll get used to it once you get the pattern indicated in this book! The same is also true of prefixes(pre-, post-, re-, dis-, ex-, etc.) and suffixes(-ed, -tion, -er, -or, -ee, etc.). Both will help you strengthen your vocabulary-building if you know what they mean.
Again, practice makes perfect!!