I'm looking for a more historical perspective on how we ended up with the orthography we have today and why it differs from pronunciation. E.g., the letter "t" does the sound originally symbolized by thorn along with its normal sound because printing presses didn't have the thorn character.
Where could I find a book or website that explains this?
I recommend David Crystal's books, and especially his Cambridge Language Encyclopedias:
There's practically no overlap between these two, which are both available in folio-size paperback. The second one has a section on the English alphabet, with a full page devoted to each letter, its history, its variants, its extensions, etc.
For total coverage, try Daniels and Bright's monumental
I recommend the seven (as of Aug 1 2018) fully accessible, popularizing books by Columbia Univ Prof. John McWhorter PhD Linguistics (Stanford).
In particular, start with Chapter 4 on p. 135
- A Vowel Is a Process: Words Start Sounding Different
, of Words on the Move (2016). It explains English's Great Vowel Shift without IPA and presuming any knowledge of linguistics.
I don't think you'll get a single contained book that explains why sounds and writing diverge or describes the history of that failing correspondence. The closest might be a perusal of any general history of the English language where all sorts of anecdotes might explain a number of spellings.
Or, one could follow a number of on-line references which do both at a linguistic level. For English, the Great Vowel Shift covers a number of vowels (but not all vowels and not all dialects).
At a practical level, such as for every single strange spelling like why 'laugh' is pronounced /læf/, an explanation would need to be discovered for any individual sound change/orthographical instance.
