The wrong cassette makes changing gear, challenging.

The customer complained that the gears on their road bike had not worked well for quite some months. On testing the gears were all over the place. A quick bit of diagnosis first. Gear hanger straight, gear cable running smoothly, chain and cassette in good condition. Nothing else bent or damaged. Then I spotted it. the owner was using an 11 speed cassette on a 9 speed groupset. That combination just isn't compatible. But how did that happen? As the customer didn't know how to change a cassette or have the tools to do so the blame immediately shifted towards the bike shop that last serviced it. I have seen some shocking errors coming out of a bike shop but not even I thought this was possible. The customer said they would use their spare bike until the cassette was replaced. Then it dawned on me! Both bikes were the same model but one had a different groupset. At some point the rear wheels had been swapped over with the 9 speed cassette now paired with the 11 speed bike and the 11 speed cassette paired with the 9 speed bike. Problem solved, no charge.

Plenty of bicycle maintenance books to choose from at https://performancebooks.co.uk/collections/bike-maintenance

We have plenty of recommendations depending on your level of skill. It may be associated with cars and motorbikes but  The Bike Book from Haynes is well laid out and easy to follow. https://performancebooks.co.uk/products/bike-book-7th-edition