Transhumanism is currently a hodgepodge of religious nonsense, visionary science fiction, and practical self-improvement. I confess I am a bit swept up in the romantic ideal of it. I love the idea of human improveability in the form of intellectual and technological advancement, extended lifespans, higher quality of life, and even post-scarcity economies.
The religious nonsense part of it is best embodied in Ray Kurzweil's singularity (also known as the nerd rapture), the idea that humanity will soon upload our minds to computers and live forever. I can't imagine us not having this technology before the end of the century--especially with efforts like the UK's Human Brain Project and America's BRAIN Initiative AND a proof of concept with researchers mapping a worm's brain into a legobot and having it "come alive". HOWEVER: I also don't pin any personal hopes for immortality on this research because we are making copies of our minds, so even if my mind joins the singularity, I will still die--probably bitterly jealous of my immortal self having all that virtual sex in technoheaven.
For me, the science fiction of transhumanism is all about vision and inspiration, and not about dreams of salvation and immortality like Kurzweil promotes. The science fiction part of it is most accessible through Star Trek, but in reality our transhumanist future will probably be more like the wild visions of Charles Stross' Accelerando, or my personal favorite the Quantum Thief Trilogy by Hannu Rajaniemi. These books drop you into settings filled with Matrioshka brains (Dyson Spheres made of computronium), and force the reader to confront all the uncomfortable otherness that comes with virtual life.
Another great science fiction resource is the Creative Commons Eclipse Phase RPG, which takes place in a future where humanity has colonized solar system and extended out into the Oort Cloud. Each planet and environment requiring different engineering and culture adaptations to survive. You can download all the books in PDF format. These books are a fantastic jumping-point for the imagining what a post-human future might look like.
This all said, I am not a fan of Sirius' encyclopedia. I was looking for practical, real-world things I can do right now to enhance my life through science and technology. Instead, I got very thin treatments of many subjects, overstatements of medical advances, important subjects left out (like the 19th Century Russian Cosmism movement (precursor to transhumanism)), and a general lack of leads to new areas to research. I get way more information from Wikipedia-surfing than I got from this book. I do appreciate his efforts though. If he gets more people into the idea of transhumanism, then more people will collaborate on it, we'll have more hacks for better living, and more people thinking about the future and human progress.