Both i dag and idag are perfectly correct and both are always accepted answers when translating from English into Swedish. The version i dag is recommended by the Language Council and it's the one we use as as the main version, but both are perfectly acceptable and widely used.
However, there is a problem with Duolingo's machinery. In the dictation exercises, they aren't able to make it accept spelling variation. So unfortunately it will tell you that you have a typo if you write idag in those.
Sand_from_Mars explained that she thought 'läsa i en metod' would work because it's really a textbook with dialogs and explanations in it, called the Assimil Method, and she says she thinks Assimil is a French publishing company.
(it doesn't work though, as I explained in a previous comment, we need to add something, For instance jag har läst det i en lärobok från Assimil 'I have read that in a textbook from Assimil'. )