Read Answering Atheism by Trent Horn or the Q&A section of WLC's website.
Honestly, atheist to agnostic is a progression, and a good one. The book Answering Atheism may help as it details practical ways to (here's those words) "accompany" someone on this journey.
In the long run, the goal is not to provide the answers, but to provide the questions; faith is not opposed to reason but is beyond it; reason can help resolve some of the issues but is insufficient to know God (lest we could save ourselves without grace and solely by "works").
The book Answering Atheism was written for just this situation; you also might like Bearing False Witness .
Answering Atheism by Trent Horn to get you started.
It's pretty cheap on Amazon:
Super deep - The Confessions
Deep but easier - I Burned for Your Peace: Augustine's Confessions Unpacked
Poletemical but really good - The Last Superstition
Similar to above but more gentle - *Answering Atheism *
Really good book on Purgatory - Leaf by Niggle
I highly recommend William Lane Craig's On Guard . He covers a number of arguments for God's existence, and he also responds to traditional atheist arguments. You could also get On Guard for Students which is On Guard intended for non-Christians. It makes a great gift. I would also recommend Answering Atheism by Trent Horn. I have not read the book myself, but others on this subreddit and elsewhere have given it good reviews.