From my English lexicology textbook:
Logically the written form should agree with the oral form. In other words, the sound should be similar to the form. This is fairly true of English in its earliest stage...
"Sound" and "form" are two very different things so saying they are similar sounds unnatural to me. I'm not sure if this usage is correct.
If you are asking if the sentence is grammatical: Yes, it is grammatical.
If you are asking if the sentence makes sense: Yes, it makes sense.
However, if you are asking if there is a better word than "similar" to use in this sentence: Possibly, but it doesn't really make much difference. I might have said something like:
The sound of the word should mirror its form.
For me the real issue is that I don't really like either "sound" or "form" in this sentence, as the more precise terms "pronunciation" and "text" would seem to be a better fit. Again, it makes little difference in context, as I understand completely what the author is saying.
I'm guessing the eventual point of this passage is that English, sadly, does not follow this rule. Not by a long shot.
