It's up to you, and it also depends on who you have. This isn't something we can answer - one student might find lectures absolutely necessary, whereas another might find the textbook sufficient.
The textbook is good, but really, most of the textbook isn't covered, so knowing what is taught in lectures would obviously indicate what would be in the exam - but the main things are: 1) you can do the homework set questions without too much issue; 2) you can do the past papers; 3) you can do the labs/online quizzes etc. The way you learn that information is up to you.
And yeah, I know it doesn't really answer your question, but you don't want to end up 1 day before the exam not knowing anything... usually the lecturer would put their slides on Moodle, so you could look at them and see if you require attending the lecture. They usually go through sample questions so knowing the method of doing certain questions is rather important, whereas it probably isn't shown in the slides.
