Personally, I suggest you learn while watching online video tutorials. This form is interactive and flexible, and it is more efficient than reading by yourself (which is boring).

But if you choose to buy self-study textbooks and learn PS by reading books, I personally recommend you to buy photoshop from Introduction to Mastery published by Tsinghua University Publishing House. This is a series of teaching materials (including other software, such as "3Dmax from Entry to Mastery" and "Auto CAD from Entry to Mastery"), which should be helpful for you to learn PS step by step.