I'd personally say, it isn't a side effect of the Lithium, it most likely is caused from the disorder. Besides being DXed as Bipolar-II(mild) 5 yrs ago, I was also DXed with mild ADD. That is my culprit, and I'm in the process of having the pdoc work with me on that besides med. treatment I am on for Bipolar.
I feel in my heart that the ADD has been a lifelong problem that no one ever DXed me with and I have been suffering with that all my life. Back in my day kids like me were just accused of not paying attention, daydreaming, where now so much more is known. The only time I did well in school (went to college in my 40's) can read,comprehend and retain was while I was on diet pills, the stimulant in them helped, I even made honors twice in a row. Amazing cause I barely passed grammar school and high school.
Dual diagnosis need to be both addressed, but for now pdoc just addresses the Bipolar
No matter what mood stabilizer I've used, the reading, concentration and focus hasn't changed, so I wouldn't say it's the Lithium. It's a fantastic med. my brother has been on it for 33+ years, and still reads, and enjoys it.
I had a professor who also was on Lithium for ages and it never interferred with his reading and hunger for knowledge.
I'm not saying it can't be your meds. but I think it's the disorder that gives us problems in that area.
See if your pdoc can offer some info and direction to help with this. I feel for you, cause it is so frustrating, I have had this happen for so long
Please take care and feel free to share any info. with us.

Sincerely,
DE