Some few more questions:
Can I use different topology to design a n order filter (e.g. LP Sallen-key and an integrator amplifier to make a 3rd order)?
If you mean by 'integrator" an RC added to a 2nd order Sallen-Key, then yes you can but there are better topologies.
I may be very wrong here, but from what I researched the type of filter I want is determined by the Q factor (Butterwhorth it's 0.707), not sure what is the Q for bessel yet, but I guess 0.58?
But using this filter wizard: http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html) I saw that when designing an even order filter the Q should the product of the Q from the second order filters is close to 0.707 and when designing an odd order filter the Q product is around 1, why is that?
(At least for butterworth)
Any good links or papers I can read on designing Bessel Filters?
In terms of filters, Q is only meaningful for a second-order denominator. For higher-order filters, that is the wrong way to think about the problem.
Bessel, Butterworth, Chebyshev, eliptic, etc, are all approximation functions derived based on certain criteria (e.g., maximally flat for Butterworth). It is difficult to know what you really want to do, but now I am guessing you want to be able to derive the Bessel function from first principles. Otherwise, you would go find it published in a gazillion different locations and just use it. I have a few books on my shelf that cover this stuff, but it is the "dusty shelf" of books written before there were calculators and likely all of the authors are dead.
In the IIR world, the LP filter design process is more or less something like this.
Define pass band and stop band parameters
Pick the approximation that meets these parameters (Butterworth, Cheby Type I, Cheby Type II...)
Get the polynomial from a reference
Break it into second-order polynomials (this can be tricky, see below)
Pick a second-order active filter topology and associated design equations for the components
The way you generated the second-order polynomials will impact component spread, noise, dynamic range, sensitivity.
In this process, if you need BP or HP, you do what is called a "frequency transformation" to convert from e.g., LP to BP
The book I suggest is "Modern Filter Design Active RC and Switched Capacitor" by Ghausi and Laker