The two books are quite different, really.
How To Improvise is essentially a collection of exercises. Each chapter isolates an extremely specific topic, and contains several exercises in order to improve that. For example, the first chapter is about pacing, i.e. deciding when to play and when not to play while soloing. The first exercises are easy: playing during two bars, resting during two bars, and so on. But at the end it gets quite hairy: for example, playing 3 bars, resting 4 bars over Autumn Leaves, with a metronome as only backing track, and not getting lost is rather difficult.
On the other hand,
Ready, Aim, Improvise is more about explaining stuff. It's much more verbose than the first book, and has a very broad scope: theory, harmony, ear training, vocabulary, practising, etc, and it even touches upon some rather philosophical issues such as dealing with one's ego. There are also plenty of exercises, but of course it's not as exhaustive as
How To Improvise in that department. It's very well written, very thoughtful, and there's a lot of wry humor.
I'd just get both books.
