I would say she is on her way to reading. Go with the flow and see where she takes you. As for your question this is not a normal behavior for not even 2 year old
DS5 was obsessed with letters since 15 months. DS4 didn't care for letters too much but he could say very early on what was the 1st and the last letter of a word (it works much better in his other language than in English).
My younger one didn't care too much about workbooks, but my older one was obsessed with them to the point that I was recommending work books as a great toy for other 2 year olds! What was I thinking? To me it was normal, to other parents I must have seemed completely crazy.
We did lots of Carson-Dellosa workbooks while his baby brother napped. Same/different was one of the first ones and also math. Most of the time I acted as his scribe. He answered the questions, but I was the one who marked them in the book. The Same/different could easily work even without your help. Your DD can mark them whatever way she wants to or even put a sticker on top of the picture instead of writing. As for simple math and such you can make her stickers with numbers or let her use stamps. Look if you can find workbooks with stickers. She may also be ready for simple computer games. I think both of my kids did pretty well with computer at that age.
Have fun!
