June Books 4) A Device of Death, by Christopher Bulis
I've finished all the Third Doctor novels now, and am onto the Fourth Doctor: here, he ends up separated from Harry and Sarah as they are leaving Skaro by Time Ring, and the three are caught up on different sides of a protracted war which turns out not to be all it seems. It's a story which has been done both before and since (most recently by Big Finish), but it's done well enough here and Sarah gets to befriend and humanise an android without pushing my I-hate-cute-robots buttons.