Hi and welcome!
You've started a great journey, I hope you enjoy it.
Your school or club must have submitted to the CAA a syllabus that they use to train you. Both AFE and Pooley's publish this. The school may have chosen to do their own, but unlikely as it costs a lot of money to get it approved.
Now, I suggest you ask your school which syllabus they are using and get the relevant manuals. In our case, we sell or recommend our students buy the manuals from our syllabus provider. We use their CD package for pre-flight briefings and all the diagrams, text etc tie in with what's in the manual.
We always discuss this with students after their Trial Lesson and they usually buy Book 1 from us, then buy the rest of the manuals on-line. We do keep the whole range in stock if people want to buy from us but generally folk like to get their stuff from a popular on-line book retailer.
One word of warning. PLEASE DON'T buy second hand unless you can guarantee the manuals are the current versions. People have failed exams by using out-of-date text books. As you say, all the books combined cost less that an hour's flying
There is also an excellent on-line learning platform which quizzes you about aero matters, but do read the text books thoroughly before using their service pre-exam.
TOO