Some assessment in the comments—that of John Lawler—is a gross exageration. True, English spelling to sound correspondence is a mess, but it is not wholly devoid of principles. There is for instance the principle that a stressed vowel before a double consonant or before a consonant cluster receives its short value; there are other such principles. For instance in the dictionary maintained by J C Wells (Longman Pronunciation Dictionary), at the beginning of each list of entries headed by a new letter, there is a page or two of guiding principles collected under sections called "spelling to sound", and it is useful to know them. Knowing them is part of the apparatus that will allow the student to acquire gradually an intuitive knowledge of English pronunciation. That is not to say that the rules will solve the questions the reader has while ploughing through his English texts, far from it, but in any case, any serious student of the language should be familiar with these principles. You will find in this book, for instance a system of pronunciation for recently borrowed French words (literary, often); in that particular case the sounds are influenced by French pronunciation, although in English they are a compromise (but it departs from traditional English).
The modern pronunciation of English has its roots in history, as all languages, except Esperanto. For instance the pronunciation of "th" in modern English is that of sounds from Anglo-Saxon, otherwise called Old English.