I don't think you necessarily need any book to work on pressure issues, least of all an expensive Arban book. You can use just about anything in your collection. Think about it.. you don't play with less pressure only when you're playing the Arban, do you? Playing with good form doesn't require a special book.. it requires the right mindset.
Let me preface by saying that I believe a certain amount of pressure is needed. I'm going on the assumption that we are talking about serious excess pressure.
Try buzzing a tuning Bb without the horn or the mouthpiece. Once you are comfortable doing that, while buzzing the note, try to place the mouthpiece on your lips without interrupting the buzz and without changing the pitch. It will be harder than you think. After while, you should be able to sustain a buzz while taking the mouthpiece on and off your lips. This will help give you not only the idea of the right amount of pressure, but it will help teach your brain that you just don't need that much pressure in the first place. Keep in mind I'm not one of those guys that buys into the old "trumpet/trombone hanging form a string and being able to play it" thing.
One possibility no one has mentioned...just a thought from out in left field..
How old are you? Is it possible you're growing more facial hair these days? I know that if I don't bother to shave for more than a day, I get really badly irritated when I play, and the ring on my lips gets pretty angry looking. A long shot, but worth mentioning.