Learning stuff involves creating new connections in your brain between different concepts. If you learn something by heart, it means the connections are very strong, and that if you think about one concept (e.g. finance), your brain automatically also activates (i.e. recalls) related information.
Most brains are pretty good at creating connections. It's very unlikely that even after coming back to the material a few months later, there are NO connections at all. It's just that these connections are not very strong, and therefore are not automatically activated. Another thing is that the brain is a network, so it often happens that concepts are connected in multiple ways than one (by going through other concepts). Basically this means, that if you're coming back to a book, you might at first not remember anything about it, but as soon as you've read a couple of lines (which activate these other neighboring concepts), it all comes back.
So how does this translate to real life steps you can do to improve it?
1. repetition. If you use a certain connection a lot, your brain will increase it's strength, meaning that as soon as one of the concepts is activated, the other is more likely to get activated as well (i.e. you recall it).
2. contextualization / intergration. Instead of increasing the strength of a connection, you can increase the amount of connections. This means to interact with the material while activating different mental concepts. For example, if you're reading about cooking, you can try to practice it. Or talk about the things you're learning with somebody. Trying to interact with the material with different 'viewpoints' (depending on the material this might or might not make sense)
3. Attention. This is kindof related to point 2; your brain has a limited capacity for active concepts, so if a large part of your brain's attention is somewhere else, it's hard to make any relevant connections. This also goes for being tired.
4. Rest. Your brain uses sleep to consolidate it's newly formed connections. So improving your sleep has a big effect on improving memory. Basically improve sleep health (fresh air, no heating, etc) and go to bed sober (no weed, or alcohol).
Because of point 2, and 3 it's better to learn something at the beginning of the day, instead of at night. I personally love to read before going to sleep, but sadly it's not the greatest for remembering.
As a general advice I'd like to add that, personally I too return to books after a couple of months, and I also often forget what they were about. I just start to read a couple of pages back, at most one chapter, if it has been a really long time. Most cases reading a small part will bring back enough, that you're able to continue. Don't get discouraged if you can't remember anything, and just allow your brain to be re-introduced to the concepts, you'll probably be surprised by how much has stuck.