Probably the best book currently available on this subject. It contains large chunks of material from the Simpkinses' earlier Effective Self Hypnosis, but adds significant new material especially regarding recent neurological evidence for hypnosis's efficacy, and some brief but interesting considerations of cognitive behavioural therapy used in tandem with hypnosis.
Some reviewers complained that the earlier book was rather vague in giving instructions for entering 'trance', but that's not really fair; though the directions here are terse, the whole point of doing hypnosis properly is not to adhere closely to the scripts one finds in the scores of useless books on self-hypnosis out there, but to feel one's way into the rather ordinary frame of mind that hypnosis is: i.e., a relatively formal analogue of the everyday experience of absorption we all go through when daydreaming, driving, or reading, (etc., etc.).
Roger Straus's Strategic Self-Hypnosis is a very good book, written from a completely different perspective. But as long as you do the exercises and find your way into a 'hypnotic frame of mind', and understand a little bit about how suggestion works, then the theoretical standpoint is irrelevant. It matters only to scientists.
I also recommend Daniel Araoz's The New Hypnosis, which is extremely well-written, and helped my understanding greatly, though it isn't a how-to as such. It's out of print, but findable.
This is the best book, written by qualified clinicians not by people with Mickey-Mouse bona fides from non-academic bodies, and is worlds apart from nearly all the other books on the subject, most of which are actually appalling.