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A deep long-slit spectrum of the diffuse ionized gas (DIG) in the edge-on spiral galaxy NGC 891 has been obtained. The slit crosses the plane at a galactocentric radius of 5 kpc and covers DIG on both the east and west sides of the galaxy. The primary motivation was to attempt to detect the recombination line He I λ5876, the strength of which relative to Hα provides a direct constraint on the hardness of the ionizing spectrum. In the DIG of the Milky Way, this line ratio has turned out to be surprisingly low (Reynolds & Tufte; Heiles et al.), implying an ionizing spectrum much softer than had been inferred from the more readily observable forbidden lines, such as [N II] λ6583 and [S II] λ6717. The He I line has been detected in NGC 891 to a height of about 1.5 kpc from the plane—well into the diffuse gas layer. In the DIG, He I/Hα ≈ 0.034, implying that helium is about 70% ionized and that the ionizing spectrum is significantly harder than in the Reynolds layer. The spectrum also allows very detailed mapping of the [N II] λ6583/Hα ratio as a function of height off the plane. This ratio reaches peak values of about 1.4 at heights of z = 2 kpc on the east side of the plane and z = 4 kpc on the west side. Beyond this height on the east side, the ratio clearly declines again, falling to about 1.0 at z = 4 kpc. Previous modeling indicates that such high values of [N II] λ6583/Hα require an ionizing spectrum significantly harder than that indicated by the He I/Hα ratio. Hence, despite the higher values of He I/Hα, the same dilemma exists as in the Reynolds layer case. These results suggest that we do not understand the heating and ionization of the diffuse gas well enough. A determination of the gas temperature would help, but the nondetection of the line [N II] λ5755 allows only upper limits to be set: 13,000 K on the east side and 10,000 K on the west side.