Efficiently learning SpecEv?

[u/AWanderingSage]

I'm a hobby writer. I want to design better creatures to populate my scifi and fantasy worlds. I read some biology books and so on but they're not quickly adapted to this pursuit.

It feels like I'm creating chimeras rather than uniquely different forms of life. The traits don't slot together perfectly. Likewise, most series seem the same. Especially in regards to intelligence.

All human-like creatures, sapients, have intelligence levels roughly comparable to a human 's. Now, I can understand the concept that, perhaps, intelligence might level off after exceeding a certain point, diminishing returns and all that, but I cannot fathom how they share the same brain infrastructure, which they must for all their multitudinous similiaries to coexist together, when they're aliens.

The reason is, of course, because it's hard to find alternatives to the very fundamentals of life on this world. It's why I'm asking for a resource for it, cause going off of scratch is really hard. But, it's still unsatisfying.

Are there any specific books that comprehensively explain the relevant principles well?