There are various themes throughout the series, though some do interweave.

I have never seen any of the books in charity shops, so I suspect that they are kept after reading.

They are not all at the same level - though which are the best ones differ for different people. The one I find funniest makes me laugh every time I read it, laugh so much that I suspect it might finish me off some time.

We lost one book somewhere in the house for some years, and found it to download onto my Kindle until it turned up again. You could go that route.

I re read the whole lot at intervals.