Cool idea doing the ecorce. Doing a human and quadruped this way is on my to-do list, no doubt. I think it would be a really nice touch to finish it off with a sculpt of the skin on top, I think it would give you a stronger indication of how accurate you’ve been.
You mentioned anatomy books… first off I would say get as many as you can. They all have different strengths and weaknesses. For me the closest to a holy grail is Paul Richer’s artistic anatomy. Very thorough and very clear - no internal organs though.
For your animal studies, I bought a huge book in Italy for my girlfriend once called “Anatomia Scuola Di Disegno” (Anatomy - School of Drawing) by Andras Szunyoghy and Gyorgy Feher. I’ve no idea if you can buy it online, but given the authors names I’d say it’s not exclusively Italian, so there’s a good chance its well distributed. Anyway, covers a LOT of animals (humans too, but the accuracy for them isnt as good as other books), and spends a lot of time comparing different species. Worth it if you can find it.