The sort of book you would recommend depends on the sort of person your friend is and the religious background he comes from. For example, Keating’s book Catholicism and Fundamentalism might be a good recommendation for Fundamentalist Baptists, but it probably would not be a good recommendation for Anglo-Catholics. Protestants are a diverse group. Keep that in mind. It might help people here if you elaborated on your friend’s background, his temperament, etc.
If your friend admires Luther so much for his deep respect for the word of God, it might be instructive to examine what Luther actually thought about the word of God. What many Protestants don’t realize is that Luther held a much more nuanced and liberal view of Scripture than they do. He was not a Fundamentalist by any means. He felt free to dispute the apostolicity and inspiration of books of the Old and New Testaments, and said that their status was a matter of private opinion. Here are some excerpts from his prefaces to certain letters of the New Testament.
Flatly against St. Paul and all the rest of Scripture, it ascribes righteousness to works, and says that Abraham was justified by his works, in that he offered his son Isaac, though St. Paul, on the contrary, teaches, in Romans 4:2, that Abraham was justified without works, by faith alone, before he offered his son, and proves it by Moses in Genesis 15:6. Now although this Epistle might be helped and a gloss be found for this work-righteousness, it cannot be defended against applying to works the saying of Moses in Genesis 15:6, which speaks only of Abraham’s faith, and not of his works, as St. Paul shows in Romans 4. This fault, therefore, leads to the conclusion that it is not the work of any apostle.
*However that may be, it is a marvelously fine epistle. It discusses Christ’s priesthood masterfully and thoroughly, out of the Scriptures, and interprets the Old Testament finely and richly. Thus it is plain that it is the work of an able and learned man, who was a disciple of the apostles, learned much from them, and was greatly experienced in faith and practiced in the Scriptures. And although, as he himself testifies in Hebrews 6:1, he does not lay the foundation of faith, which is the work of an apostle, nevertheless he does build finely thereon gold, silver, precious stones, as St. Paul says in 1 Corinthians 3:12. Therefore we should not be hindered, even though wood, straw or hay be mixed in with them, but accept this fine teaching with all honor; though to be sure, we cannot put it on the same level with the apostolic epistles.
Who wrote it is not known, and will not be known for a while; it makes no difference. We should be satisfied with the doctrine that he bases so constantly on the Scriptures, showing a right fine grasp upon the reading of the Scriptures and the proper way to deal with them.*
Concerning the Epistle of St. Jude, no one can deny that it is an extract or copy from St. Peter’s second epistle, so very like it are all the words. He also speaks of the apostles as a disciple coming long after them, and quotes sayings and stories that are found nowhere in the Scriptures. This moved the ancient Fathers to throw this Epistle out of the main body of the Scriptures. Moreover, Jude, the Apostle, did not go to Greek-speaking lands, but to Persia, as it is said, so that he did not write Greek. Therefore, although I praise the book, it is an epistle that need not be counted among the chief books, which are to lay the foundation of faith.
Finally, let everyone think of it as his own spirit gives him to think. My spirit cannot fit itself into this book. There is one sufficient reason for me not to think highly of it,-Christ is not taught or known in it; but to teach Christ is the thing which an apostle is bound, above all else, to do, as He says in Acts 1:8, “Ye shall be my witnesses.” Therefore I stick to the books which give me Christ, clearly and purely.