1. 'To be buried in books' means to have lots of reading to do. Students are usually 'buried in books'. Of course it is figurative, not literal. I think it is a great idiomatic phrase, and really gives a good visual of someone being covered in books.

a. My poor son is buried in books at the moment, as he has a science exam tomorrow.

b. I am swamped! I'm buried in books, and I need a break!

2. 'A mix up' really means a confusion and a problem. It can be used in any context.

a. There was a mix up at the airport, and I ended up with someone else's luggage.

b. There was a mix up at the restaurant, and I received the bill for the party of 30 people!

3. 'I have to keep track of the reading requirements'. To keep track means to pay attention, to stay on the correct path, to remember.

a. It's your responsibility to keep track of what you spend.

b. Let's keep track of her illness to see if she improves or not.

4. 'To be back on track' is related to 'to keep track of'. We use this phrase when we have returned to a desired routine.

a. I'm back on track with my running; I jog with my friend three times a week.

b. Now that I am over the flu, I'm getting back on track with the household chores.