I am in the process of deciding on new hardwood or engineered floors for a 4 level townhouse I recently bought. The dilemna is since the home is fairly narrow and the joists run front to back, is what direction to run the floors; the subfloor is 5/8" plywood and the joists are 2x10 16" OC and sistered about every 4th joist. If I ran the floors perpendicular to the joists the floor would look very choppy in the halls and it would probably make some small rooms look even smaller. Also the natural light comes in front to back so it might reflect badly in some areas.

I am wondering if adding 1/2" plywood would be sufficient to reinforce the areas I need to run floors parallel to the joists. Are there any other suggestions for dealing with this house. I don't want to go diagonal as it wouldn't look very good in some areas.

You are on the right track. For a 5/8" thick plywood subfloor installed over joists 16" oc, one needs to install either perpendicular or diagonally to the floor joists. OR, adding 1/2" plywood will stiffen up the subflooring to allow one to run the flooring parallel to the joists. This would be especially important for a thin, glued down engineered. Or any thinner flooring as thin flooring adds little structural reinforcement, whereas solid 3/4" strip flooring run at 45 to 90 degrees to the joists adds a considerable structural element.

I misstated the subfloor; it is currently 5/8" OSB, not plywood. Will a 1/2" reinforcement of plywood or OSB be sufficient.

1/2" plywood, glued and well stapled (or screwed), over your existing OSB subfloor, will allow you to run your flooring parallel to the floor joists. There is one caution here however. Prior to installing the 1/2" plywood, check to see how flat the subfloor is now. If you have joists that are high or low, the best time to fix them is prior to installing the plywood.