Solution
1. Nd6 Bxd6 (if 1...Ra8, for example, then 2. Nxf7+ Rxf7 3. Rxf7 Bxf7 4. Rxg7! Be6 5. Qh6 wins) 2. Rxg7 Bc5 3. Qh6 Bxd4+ 4. cxd4 Bf5 5. Rg5 f6 6. Rgxf5 Qxa4 7. Rxf6 Qxd4+ 8. Kh1, 1-0

Nunn rates this problem difficult as a 3 on his scale of 1-5, with 1 being easy and 5 being the hardest.

Here is the PGN.

Starting observations:
- material is equal
- black is attacking white's knight
- rest of white's position looks solid/not under attack or subject to tactics
- black's king isn't super well defended, g7 in particular is weak while white has 1 heavy piece on it and the bishop has a potential discovered attack

Nd6 with the threat of nxf7 looks strong. E.g. something like:

1. Nd6 Rb8 2. Nxf7 Bxf7 3. e6 Qxe6 5. Bxg7+ Kg8 6.Bf6+ leading to mate.

Not sure about other defenses but it looks like the Nxf7 sac is good to me.

Therefore black has to remove the knight with his e7 bishop:

1. Nd6 Bxd6 2. exd6 f6

Then I don't see anything better than Bxb6 Qxd6 and black has a pretty weak pawn structure and white can continue to build up pressure vs. the dark squares on the kingside while black kind of just has to sit there.

Hmm, I missed Rxg7 before moving or capturing with the e5 pawn, taking advantage of the fourth point I made when I started. Ooops.