@vclugoar hey!
Still working on the book app! That’s great 
Seriously, your app is looking AWESOME! and the rows of recommended book covers seems to work perfectly! Love the styling and all the visuals!
(hey did you see that you can now make your own theme with custom theming?! That might be right up your alley for this. You just need to upgrade to the newest Streamlit version, check out the post here: Announcing Theming for Streamlit apps! 🎨)
Ok enough of my chit chat and lets get your question answered 
So this is an easy fix, and is actually a pandas issue, your line here:
This is actually overwriting your pandas dataframe each time someone does this, its not appending the data at the end of the database. To append is a super quick fix though. You should just be able to directly append that new data on to your df. (Note: I believe you will need to create df as a pandas data frame before you run this, so if you run into an error that could be why)
df = df.append(pd.DataFrame(data=d))
here is the pandas docs if you needed more info on this: https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.append.html
Happy Streamlit-ing!
Marisa