I Dreamed of Africa by Kuki Gallmann (1991) — Gallmann's memoir of leaving Italy to farm in the Laikipia plateau, north of Isiolo, is one of the most vivid accounts of northern Kenya ever written. Loss, wildlife, and the pull of a particular landscape. Essential reading for anyone heading into this part of the country.
My Kenya Days by Wilfred Thesiger (1994) — Thesiger spent decades travelling the northern frontier districts of Kenya, including the country around and beyond Isiolo. His late-life memoir captures the rangelands, the peoples, and the landscapes of the north with the eye of someone who walked it on foot and camped with the communities who live there.
Weep Not, Child by Ngugi wa Thiong'o (1964) — The first novel published in English by an East African writer remains one of the most important. Set during the Mau Mau uprising in the Central Highlands, it provides essential context for the Kenya that Isiolo sits at the edge of — the green, historically contested highlands you've just come through.
A Field Guide to the Larger Mammals of Africa by Jean Dorst and Pierre Dandelot — Not literature, but indispensable before entering Samburu. The reserve contains several species rarely seen elsewhere in Kenya (reticulated giraffe, Grevy's zebra, Beisa oryx, gerenuk), and knowing what you're looking at enriches the experience considerably.