Excitation energies of the $ns$, $np$, $nd$, and $nf$ ($n\ensuremath{\le}$ 9) states in Li-like Be${}^{+}$ are evaluated within the framework of relativistic many-body theory. First-, second-, third-, and all-order Coulomb energies and first- and second-order Breit corrections to the energies are calculated. Two alternative treatments of the Breit interaction are investigated. In the first approach, we omit Breit contributions to the Dirac-Fock potential and evaluate Coulomb and Breit-Coulomb corrections through second order perturbatively. In the second approach, we include both Coulomb and Breit contributions on the same footing via the Breit-Dirac-Fock potential and then treat the residual Breit and Coulomb interactions perturbatively. The results obtained from the two approaches are compared and discussed. All-order calculations of reduced matrix elements, oscillator strengths, transition rates, and lifetimes are given for levels up to $n$ = 9. Electric-dipole ($2s--np$), electric-quadrupole ($2s--nd$), and electric-octupole ($2s--nf$) matrix elements are evaluated in order to obtain the corresponding ground-state multipole polarizabilities using the sum-over-states approach. Recommended values are provided for a large number of electric-dipole matrix elements. Scalar and tensor polarizabilities for the $ns$, $n{p}_{1/2}$, $n{p}_{3/2}$, $n{d}_{3/2}$, and $n{d}_{5/2}$ states with $n\ensuremath{\le}9$ are also calculated. The scalar hyperpolarizability for the ground $2s$ state is evaluated and compared with the result of a nonrelativistic calculation.