The metallicity of galactic gaseous halos provides insights into accretion and feedback of galaxies. The nearby edge-on galaxy NGC 891 has a multicomponent gaseous halo and a background active galactic nucleus (AGN; LQAC 035+042 003) projected 5 kpc above the disk near the minor axis. Against the UV continuum of this AGN, we detect lines from 13 ions associated with NGC 891 in new Hubble Space Telescope/Cosmic Origin Spectrograph spectra. Most of the absorption is from the warm ionized gas with log T = 4.22 ± 0.04, log n H = -1.26 ± 0.51, and log N H = 20.81 ± 0.20. The metallicity of volatile elements (i.e., C, N, and S) is about half solar ([X/H] ≈ -0.3 ± 0.3), while Mg, Fe, and Ni show lower metallicities of [X/H] ≈ -0.9. The absorption system shows the depletion pattern seen for warm Galactic diffuse clouds, which is consistent with a mixture of ejected solar-metallicity disk gases and the hot X-ray-emitting halo (Z = 0.1-0.2 Z ⊙). The warm ionized gases are about five times more massive than the cold H I-emitting gases around the galactic center, which might lead to accretion with a mean rate of 102 M ⊙ yr-1 for a period of time. We also detect low-metallicity (≈0.1 Z ⊙) gases toward LQAC 035+042 003 at 110 km s-1 (a high-velocity cloud) and toward another sight line (3C 66A; 108 kpc projected from NGC 891) at 30 km s-1. This low-metallicity material could be the cold mode accretion from the intergalactic medium or the tidal disruption of satellites in the NGC 891 halo.