Aptitude tests are not necessarily based entirely on aptitude.
They can be weighted in different ways for the same test, for example you may be given 'marks' for doing well, but you will also be given as many (if not more) marks for showing consistent improvement as you take the test. They can be weighted for speed, or for accuracy, or for consistency...
At the end of the day, they are only computer algorithms and have as many shortfalls as benefits. As has been mentioned, many an experienced pilot has been told they don't have the aptitude, despite having 20k+TT. Many tests for example appear to penalise smooth but assertive operation during the eye hand coordination tests, despite this is the way pilots are taught to fly, and this is one possible reason for multiple failures - put another way;
The tests are tailored towards fresh bods with little or no flying experience, who are generally young and energetic. Therefore the tests mark quick (and overly?) assertive reactions highly, whereas the more controlled and smooth approach taken by the experienced older captain will be marked down, despite being a more comfortable solution.
These tests are a massive can of worms once you start getting into them, so much so that it really is pointless trying - each of them is different.
Which is why emphasis is (or should be) also put on the personal interviews and other excercises, I can promise you that you would have to do something spectacular to fail a selection process on aptitude test scores alone!
Climb 350: Danny212 is right, just do little bits now and then. For example counting up your shopping bill as you do your weekly food shop, or working out your fuel efficiency and cost per mile as you drive, or estimated time of arrival given your speed and distance remaining... how will that be effected if you have to travel 10miles at a lower speed.... how will that affect your fuel... How much has the trip cost...
No books or courses needed for the mental arithmetic side at all!