I can understand people really adoring the in depth details and cutaways into ships, machinery, weaponry; Like Xenomrph said the Blueprint book is coming out. But no one seems overly keen to cut into a Xenomorph and see what they can find (maybe because it takes away the mystery?)
I think a lot of it is because it takes away the mystery. The USCM Tech Manual has a whole chapter about exploring Alien biology, but the end result of the chapter is that no one knows anything new and stuff is more confusing than when it started. That wasn't done by accident.
Likewise, the comic series 'Aliens: Labyrinth' literally has a scene where they dissect an Alien's head and talk about its biology, but then later in the story the Aliens are shown doing totally insane shit to Church's family that turns everything upside-down.
I think a lot of the reason why people reacted poorly to 'Alien: Covenant's idea that David may have "invented" the Xenomorph is that it ruins the mystery - it takes the Alien, an unknowable space-beast with impossible biology, and turns it into something anyone could create in a lab given the proper tools. Sure, the specifics of the Black Goo and how or why it does what it does are still huge question marks, but it still undermines the Alien in the same way that a caveman conquering fire does - the caveman might not understand the specific chemical and physical processes behind the creation of fire, but the fire is still less scary and mysterious because he knows he can create it at will.
I'm not saying a detailed art book exploring Alien biology is a bad idea - the Anchorpoint Essays is a cool website, after all, and talking about this stuff can be interesting and lead to neat creative tangents. There's just a really difficult and really important balancing act of showcasing the Alien's incredible and mysterious biology without compromising that mystery and turning it into the Veterinary Anatomy Coloring Book.
Not to mention, there's a lot of technical challenges - these are ultimately fictional creatures, and their designs and abilities vary from movie to movie (and that's if you restrict yourself to only the movies). That's often due to deliberate storytelling reasons, or technical limitations of the props, or visual design decisions. You can spend the rest of your life trying to reconcile the changes from movie to movie, but broad, sweeping generalizations like "it came from a dog" or "they're older" or "they're genetic aberrations due to cloning" to try and explain every change tend to fall apart under any sort of close scrutiny, not to mention it assumes 'Alien' is the "pure" baseline and everything else deviates from it, and not the other way around.
So would this hypothetical biology book only look at the Alien design from one movie? If it looks at multiple movies, does it even bother trying to reconcile the differences? If anything, I think it should play those differences up, emphasizing that the Alien doesn't have a singular set design.
The book is out of print, but if you can track down a copy of the Warhammer40k reference book Xenology
, I highly recommend it. Aside from the artwork being super cool and the meta-narrative being a lot of fun, it does a great job of doing what I think a good "Alien Biology" reference book should do - it explains some things in purely biological terms that are understandable, but has a lot of surprises and unexplainable stuff that still keep the alien races interesting. If I were putting together an Alien Biology book, I'd absolutely use "Xenology" as the benchmark in terms of tone and content.