I'd go with Dirty Pair by Adam Warren.

A thought: Are you fixed on explicit Transhuman themes in scifi? I ask because tropes and topics that could be considered Transhuman are surprisingly common in modern fantasy books, including one of my all time favorite quotes on the subject; "How do you decide if someone is human?" "I don't. It's not up to me to assess someone's humanity. Being human in our world is synonymous with being included into the framework of society. Humanity entitles one to certain rights and privileges, but also implies voluntary acceptance of laws and rules of conduct. It transcends mere biology. It's a choice and therefore belongs solely to the individual. In essence, if a person feels they are human, then they are." - Ilona Andrews, “Magic Bleeds”

The Ophiuchi Hotline by John Varley, a rarely discussed influence on EP. Features a transhuman civilization living in the ruins of the Solar System after an inhuman intelligence has rendered Earth uninhabitable, including a conservative Lunar government with a leader who strongly advocates for the reclamation of the homeworld.

Since you're focusing on Rifters (so good), I assume you've read Blindsight & Echopraxia. Watts is a cool guy, he's great to talk to at a con and his panels are fantastic (e.g. how Rule 34 can save the world). I'll second for Varney. He wrote eight short stories and a few novels (Ophiuchi Hotline and Steel Beach) set in a transhumanist post-apocalypse. For things not yet mentioned: Beggars in Spain by Nancy Kress is about the characters who tread the boarder between two species of humans created through genetic modification: Sleepers and the Sleepless. It explores compassionate capitalism, extra-territorial orbitals and basilisk hacks. Poseidon's Children by Alistair Reynolds transhumanism, forking and ETI explored. Focus is on the autonomous aquatic nations, who have engineered themselves into mermen and merwomen. Individuality, reality and personhood are explored in the first work, the second focuses heavily on identity, as well as fatalism and existential destiny. Ancillary Justice by Ann Leckie is a must read. I'd also recommend Greg Egan's short fiction, specifically Wang's Carpets.

Adam Roberts - Stone The protagonist is the sole murderer in a far-future transhuman society. http://www.goodreads.com/book/show/71247.Stone

The Death's Head series by David Gunn has a gun smarter than the main character, and factions of transhumans. The Parrish Plessis trilogy by Marianne de Pierres is more postcyberpunk but the titular character is extremely enhanced.

A mix of biopunk and fantasy, The Dark Lord of Derkholm - it's good fantasy, and it perfectly captures the surreality of a family where half the members are humans and half are gryphons. I suppose you might actually have that happen on Fortean, come to think of it. More on point, William Gibson is a great place to start, as it's where the genre did. Ghost in the Shell is a good way to immerse yourself into a world of ubiquitous cybernetic augmentation, and Deus Ex: Human Revolution makes transhumanism vs. bioconservatism a core plot point of the game. The Maximum Ride series is an interesting YA-fiction example of transhumanism's tropes, but in a very Deus Ex "I never asked for this" sort of way. Another example about teens with wings is the book "Flight" by Neil Hetzner - it's a very dark book, ironically, but genetic augmentation is something of a rite of passage by this point in history.

I just had my saves wiped out, and I was 1/3 of the way through it, but I remember I had conversations and in-game literature discussing the concept when last I played it. It's definitely more cyberpunk, however, but I think that's something of a false dichotomy.