falsification :
Weak players only analyse continuations in a combination that are favourable to them, I go here he takes the Rook I take back etc.
The problem is he might not take that rook but go say Ra1 #.
You must analyse your move and try to prove that your assumptions of the reply are false, try to find the best reply for your opponent.
In Blitz and speed problem solving you can miss this step out due to time pressure and it can become a bad habit OTB.
To learn the habit of falsification is what seperates the strong player from the weak.
On the point of positional vs tactical chess, if there is such a thing, if you sit down two players one playing solely positionally and one playing solely tactically the tactical player will score 100% that is why you start with tactics.
The book I use is Chess: 5334 Problems, Combinations and Games by Polgar. Most of it consists of mates-in-two, but towards the end, there are very long and difficult combinations. It should provide excellent long-term practice, i.e. probably something like a year's worth, depending on how much time you spend on tactics.