Reddit mentions of The 36-Hour Day, sixth edition: The 36-Hour Day: A Family Guide to Caring for People Who Have Alzheimer Disease, Other Dementias, and Memory Loss (A Johns Hopkins Press Health Book)

These two books:
https://www.amazon.com/Learning-Speak-Alzheimers-Groundbreaking-Approach/dp/0618485171
https://www.amazon.com/36-Hour-Day-sixth-Alzheimer-Dementias/dp/1421422239

I commented elsewhere here but I need to add that you should read The 36-Hour Day. Also visit the Alzheimer's Association website. (It supports all kinds of dementia, not just Alzheimer's). On the latter, click "In My Area" to find help in your dad's area and perhaps in your own. Many areas have local caregivers support groups.

My mom (late 70s) is well down the road with dementia and is now in a memory care unit of a nursing home. She often asks about her parents and asking when she will get to 'go visit' them. Rather than replying blountly that they are dead and have been for years, my siblings and I just say "Mom, grandpa would be 110 years old if he were still with us." Then she smiles it off. It doesnt do any good to keep correcting her when she doesnt make sense.
There is a book called 'The 36 Hour Day' that offers good advice for those dealing with those affected by dementia/alzheimers.

https://www.amazon.com/36-Hour-Day-sixth-Alzheimer-Dementias/dp/1421422239/ref=mp_s_a_1_3?keywords=34+hour+day+book&qid=1574269564&sr=8-3

I do recommend an Alzheimer's support group for you. www.alz.org. You need the empathy, you need to know what resources are out there, current treatments and you need a break. Meetings are usually once a month and are for family and friends.

I also recommend reading: The 36 Hour Day. Its a classic.