I was #1 on the 9x9 ladder for a few weeks. If you’re black, start at A, B, C, D, or E. Avoid X, which takes too little territory and exerts too little influence on the other corners.
After move 1, the branching factor really hurts the ability to study continuations. When I tried several times to write a book about 9x9 openings, I gave up. There’s too many. If Black plays B, for example, White has about 21 satisfactory replies.
Luckily, you don’t need encyclopedic opening knowledge to succeed at 9x9. The general strategy is to create just one or two groups (a third will almost certainly die). And don’t be too hasty in creating a second group (The first one is good enough more often than you might think.). Be patient, and keep your group(s) strong and connected. But don’t be afraid to throw your weight around either. A lot of 9x9 is like hand-to-hand combat.
Feel free to ask me about any specific variation.
Here’s some resources you might enjoy:
Go 9x9 Opening Explorer (yannikkeller.de)
Waltheri’s go pattern search - 9x9 games
The 9x9 Amateur-Hour Strategy Thread - Go Resources - Online Go Forum (online-go.com)