  We obtain a sharp result on order of certain affine and linear invariant
families of harmonic quasiconformal mappings with bounded Schwarzian norm. This
problem is motivated by the work of Chuaqui, Hern\'{a}ndez and Mart\'{i}n
[Math. Ann. 367: 1099--1122, 2017]. Firstly, for $K\ge1$, we construct a
harmonic $K$-quasiconformal counterpart of the classical Koebe function and use
it to formulate the corresponding conjectures. Then we consider Hardy spaces
$H^p$ of harmonic quasiconformal mappings by applying results for
quasiconformal mappings obtained by Astala and Koskela [Pure Appl. Math. Q. 7:
19--50, 2011]. In particular, we determine the optimal order of the family of
harmonic quasiconformal mappings with bounded Schwarzian norm to belong to a
harmonic Hardy space. This partially solves an open problem posed by
Pavlovi\'{c} in 2014. Finally, we derive pre-Schwarzian and Schwarzian norm
estimates of certain harmonic mappings.
