Reading Bbefore Ssleeping

It seems more normal these days for titles to be lowercase. Your version is correct, just less common.

I always take notes withon my phone while I'm reading.

'With' isn't wrong, but suggests you might use the camera, or voice notes. 'on my phone' suggests you write notes.
Slightly more natural: "I always take notes on my phone when I read."

ButThat said, I try not to expose my eyes to blue light before bed.

Or: "I always take notes on my phone when I read, but I also try not to expose my eyes to blue light before bed."

Therefore, I'm only down for fictions books, which I rarely take notes on, when I'm reading in bed before going to sleeping.

Fiction, not fictions. e.g. romance, detective, fiction, factual, etc.
"I'm down for" is very casual. Fine in spoken English, but a bit odd in written English.
These all work, too:
Therefore, I'm only down for fiction books, which I rarely take notes on, when reading in bed.
Therefore, I'm only down for fiction books, which I rarely take notes on, when reading in bed before going to sleep.
Therefore, I only read fiction, which I rarely take notes on, when reading in bed before going to sleep.

Therefore, I'm only down for fictions books, which I rarely take notes on, when I'm reading in bed before sleeping.