That's right, but it means something a bit different. You might also say "I was interested in reading his report", but that means you had the interest, but you may or may not have ever read the report in the end, whereas "I was interested to read his report means that you read the report, and what you read was of interest to you. Also, you may or may not have been interested beforehand, so you could say something like "I hate reading reports, but I was interested to read his, because it seems sales of widgets in Upper Patagonia went up 100% in the last quarter". But note that you can't use this construction where that meaning makes no sense; you can't say "I was interested to go swimming", for example.

Thank you!