I am not aware of any substantial, English-language biography on Omar Pasha. This is most unfortunate as he led an intriguing life full of adventure and almost unparalled success. He is mentioned quite often by British soldiers in the Crimea of course but little is recorded of his many later campaigns.
Part of this may be due to the perceived excesses he ordered/allowed when he was recalled to put an end to the Cretan revolt in 1866-68. Western newspapers, already somewhat biased in favour of the Greek cause on the island, presented a somewhat unbalanced view of the war. Neither side had a monopoly on atrocities but the final Ottoman solution to end the revolt was medieval in its cruelty and detracted from the hard-won laurels which Omar had garnered during most of his military career. Perhaps Western readers didn't want to be reminded that he had been born and educated as a Christian?