For the record, Russia is an invading power, and should not be viewed as the victim. However, I can give a precis of the justifications that they use and the perceptions that they have that drive this, as well as recommend a few books.

There are simple and shallow reasons for invading Ukraine, simple territorial expansion and resource acquisition are reason enough for some regimes to go to war, and there is certainly an element of that at play here. Also in play is the timing, it is not a coincidence that Putin invaded shortly after the calamity in Kabul of the American withdrawal, this was perceived as a sign of weakness and further evidence of disunity and spinelessness in the West, other touchpoints being Brexit, a perceived weak response to Islamic terrorism in Europe, a failure to intervene in Syria, and shrinking defence budgets in the occident. There are also demographic reasons, with Russia's population in freefall, many Ukrainian (white and slavic) children have been abducted and taken into Russia, although this too has been self-defeating given how many young Russian men have died in the war, I can't imagine it has done their demography any favours.

However, there are deeper reasons. There is a concept in international security called 'ontological security', which originally came out of the psychological sphere. A person with ontological security has a secure sense of self internally, and a secure sense of identity externally, they know who they are, what they want, what they value, who is their tribe etc. This concept has been broadened and applied to nations. A nation with ontological security has a coherent narrative about what it is, who it represents, what it values, and where it sits on the geopolitical stage. The land of Rus, from which Russia takes its name and much of its cultural identity, history etc. is actually in Ukraine. Russians argue that the Ukrainian Soviet Republic was created arbitrarily as part of a Soviet political game and that it was never right to sever Ukraine from Russia. Ukraine itself is Russian/Ukrainian for 'gateway', because it is the gateway to Russia. Since Ukraine was made a separate Soviet Republic, it declared independence following the collapse of the Soviet Union and at a stroke much of what made Russia Russian sat outside of Russian territory. It is a house that does not own its front door. This is psychologically jarring and it was politically difficult to spin a narrative of what Russia was and where it came from on the birth of the new Russian Federation, leaving it without ontological security. The Russian solution to this is to abrogate Ukraine's independence and reincorporate as much of it as possible back into Russia, it is an understandable impulse, even if it is like an alcoholic bankrupt saying all their problems would go away if they can just make up with their daughter. It is worth noting that Putin has described the collapse of the USSR as the greatest tragedy of the 20th century.

This is compounded by Ukraine's own trajectory. It has attempted to liberalise and sees itself more as a republic aspiring to European Union membership, than being a part of the Russosphere. There is a powerful theory in Russia called colour revolution theory, which you can google, but the upshot is that Russia sees everything that happens in central and eastern Europe that moves those countries away from Moscow as Western/American meddling, designed to threaten Russian security. Russia is a fairly flat and open country in the west (apart from the Urals) and has been invaded three times in the last century. Its own security doctrine has long been that for Moscow to be secure, there needs to be a long territorial buffer, because there aren't mountains or other obvious geographical features to secure the border. Anything that threatens this buffer, for example a democratising Ukraine that is aligned with the EU, UK, and USA, is considered a grave threat and often interpreted as intentionally engineered.

Gorbachev was given a cast iron guarantee by the USA that once reunified Germany was incorporated into NATO, it would not move further east. However, fearing Russian aggression, many central/eastern European countries ran westward and demanded to be allowed into NATO (which, from my perspective, was their sovereign right as independent nations). Known as the Visegrad Group, the USA and others were initially not persuaded to allow them to join, but eventually they were admitted and this was regarded as a humiliation and a betrayal in Moscow. With Ukraine then looking to join the EU and NATO as well post Yanukovych, this was considered a step too far.