Now to your question: A to-be-or-not-to-be interrogative issue.
If this is the case: 今天我看了几本书。(rather than 今天我看了几本书？)
I wouldn't say it's so wrong, personally, but it's not so right, either (haha!)
今天我看了几本书。can be like a statement but an unfinished one though.
More info shall suffice for it.
I read some books today, which are (maybe here 'were' better?) about
Brazilian and Vietnamese coffee beans. I was so into it
that I would start a business straight away!
I flipped through some books, got some bites of biscuits,
and then went back to bed again.
Yes, 今天我看了几本书。does sound like an interrogative,
but when the subject 我 gets replaced by 你 it will make an interrogative 100%.
How may books you read today?
You read some books today.
How many books I read today?'
This one sounds like a Jack guy being asked,
'Jack, what is your name?' by an Alzheimer's patient.
I mean, how possibly I couldn't know how many books I read today,
not even a rough number I can recall for it.
Today I read some books.
(makes sense but sounds incomplete somehow)
How many books I read today--you ask me?
How many books he read today--you ask me?--I ask whom?
How many books I read today? Hmm, let's see, I read... blah blah blah
Mind ElleLee's words, and also the definition '二' in anita998710's.
They both gave out an insight that 几 has got yet occasionally ignored.
Take the original piece, for example: 我今天看了几本书。
I would think you're saying 2 to 4 books, probably. Or maybe 7.
9? Well could be.. but err, 9 within a day?! You must be a superwoman
or talking about children books, most of which I see only pictures (haha!)
Also, you would ask an under-10-year-old kid 你几岁了？ indead
Not sure if you already knew to make sentences such as:
我今天看了三、四本书。(here it will be only a full stop '。' without question)
I read 3 to 4 books today.
I read 5 to 6 books today.
These are very common ways of expression,
using two neighbouring integrals for a rough number under 10 exclusive
because of 九、十 'nine and ten' making it like 九十 'ninety' in speech.
Well in addition to 几本书, you could practically use 一些书 and 若干本书
as well (both which I will absolutely not think you're questioning or
soliloquising in any case) for 'some books', well if you want to stick with
the teacher's viewpoint and play safe.