Wildly inventive time travel always makes for fascinating sci-fi novels, and “Permafrost,” from the pen of renowned Alastair Reynolds, crackles with ingenious plotting. Set in the frigid wilderness of the Arctic in 2080 and 2028, a future Earth, poised on the precipice of doom, sends an elderly teacher back into the past to tweak their present. Valentina Lidova is an engaging heroine and the book’s early pages, chock full of character twists and explanatory time travel science, make for sprightly, intelligent reading. At 176 pages, novella length, I felt the the necessarily complex plot swamps any sense of story, and towards the end, when AIs are introduced, my involvement waned. So … intriguing ideas and a believable protagonist make “Permafrost” a short but perhaps uninspiring futuristic read.