"He was able to read."
Translation:Il a pu lire.
How common is this phrase, please? I had expected a combination using the word "capable" but instead got "He had the power to read".
"Il a été capable de lire" and "Il était capable de lire" are also correct and accepted translations.
The passé composé ai pu / as pu / a pu ... etc would be very commonly used. The word pouvoir being used here is the verb meaning "to be able". There is also a noun pouvoir which means "power", for example au pouvoir = "ïn power". Remember, verbs have past participles and can be conjugated, nouns cannot.