If you love etymonline.com for exploring the history of words, here's another rich resource, a dictionary of British English spelling (one dialect only) arranged by phonemes with their corresponding graphemes (for spelling) and graphemes with their corresponding phonemes (for reading).
It's not a lightweight book and, like etymonline, contains more information than most people need, but it contains the knowledge required to help us make wise choices for and with our students when teaching spelling.
What are the most common ways to spell a sound?
What are the less common ways but in common words?
Which graphemes can be left until the need arises?
I use the dictionary to calm anxiety by highlighting the predictability of the English code for spelling. A teacher asked how phonics can help with spelling when there are so many ways to spell the same sound. She then offered the following options for spelling /f/:
f
ff
gh
ph
It's a fair question and my answer was to turn to the dictionary and ask the group what they thought might be the frequency for those graphemes.
Have a go at the quiz below. Flip the box for a link to the answer as found in the dictionary.
Graphemes for /f/
Question 1
What are the three main ways to spell the /f/ sound in English?
What do imagine their frequency to be?
Graphemes for /f/
Question 2
How many less frequent spellings of /f/ can you think of?
Are they all essential for your students?
Those answers calmed the teachers' worries. There are lots of ways to spell /f/ but many of them don't need to be taught until the need arises or unless they appear in very common words like 'cough' or 'rough'. Teachers and students can decide together which of the obscure graphemes are most useful.
Everyone needs to know:
the three most common ways to spell the sound
the patterns for when and/or where those spellings are likely to appear
that there are other graphemes you can learn as you need them