You can find French books in some places, but you can find cheap Portuguese books virtually anywhere, but particularly so in Porto, close to where I live.

We call local/old/second hand bookshops “alfarrabistas” and they are very common around the country. Some have closed with the savage turistification that’s hit the country, but most are still around for now.

Do you have any favorite you feel like suggesting? I've been there several times but never noticed them and even my Portuguese partner wouldn't know where to find one after many years living abroad.

Where? In Porto or Lisbon?

Both are fine :)

If you are moving there please make an effort to support the local economy and get involved with local politics and governance. The country is being flooded by techies and rich artists thinking that Lisbon is the new Berlin while ignoring completely that they are considerably damaging the local population’s quality of life.