If I may add a comment or two to MFS's post.
(a) do run some searches in this site as these questions, quite naturally, come up time and time again as new people become interested in the site and finding out more about flying.
(b) Endurance basically means looking for where we can run the aircraft with minimum, or near minimum, fuel flow (ie maximum time available for loitering flight) and it works out that pistons like low and jets like high. The specific height chosen will depend on the individual aircraft to some extent and how you go about the analysis .. what factors you include and so on. Some jets will give best endurance at a medium height (eg earlier 737s typically somewhere around FL200) .. it all comes back to doing the sums on the specific aircraft data. The generic "go high" story is only that .. generic... however, the jet does not like low ... you often find fuel flows at the holding point to be not much different to what you get at normal cruise levels and speeds.
(c) stall speed variation with altitude often confuses people. The typical lift equation we learn as pilots
lift = CL x 0.5 x rho x V-squared x S
can be reworked to something like the following to approximate the one G stall case which occurs at the maximum value of CL (CLmax) ... if this is not in your principles of flight book, then it probably is in Kermode or Davies (long time since I have read those texts) or else check out some of the websites listed in the Tech Log sticky thread.
Vs = root ((2 x W)/(rho x S x CLmax)).
This is all fine but you MUST always remember that real world processes are complex and, as engineers and scientists, we have to approximate things and make various assumptions to be able to get whatever the job is done ....
This "standard" equation that you will see so often makes the assumption that the CL curve only depends on alpha, which is reasonably accurate over a small altitude band, typically near sea level for common use. However, there are other things which come into play. Two considerations, which typically we ignore for the near SL case are Mach Number and Reynolds Number which are very important fluid flow measures. As an aircraft climbs to much higher levels these measures change with their effect on the CL curve becoming measurable and having to be considered.
In regard to stall, they reduce the CLmax value which the wing section is able to generate. If you revisit the equation above, if the value of CLmax is reduced, then the right hand side of the equation gets bigger in value and stall speed increases somewhat. In practical everyday terms it is not something to get ulcers about as the book data to which we fly the aircraft considers such matters for us.
And you need to keep in mind that the ASI is a differential pressure gauge calibrated to give an indication of speed at standard SL ... it is not a speedometer like you might have in your motorcar. Except for standard conditions at SL, the ASI tells fibs ... which is why we have all these various "airspeeds" to worry about ... they allow us to account for the various fibs which the ASI is wont to tell. Once you get used to it, it all seems to fall into place. In practical terms, we are concerned with what the gauge tells us (IAS) for operating the aircraft (for jets we need the Mach Number as well) and the TAS we calculate from that for figuring out the navigation solution. EAS is important for engineers but much less so for civil line pilots.
(d) The jet engine operates effectively only over a small RPM band due to the problems involved in getting the air to find its way through the machinery .. just consider all the little wings you have spinning around and the torture to which the airflow is subjected. The typical thrust curve shows very low thrust at low rpm, slowly increasing with increasing rpm and then rapidly increasing in the higher rpm range. These turbomachinery flow problems make turbine engine design a pretty specialised engineering discipline and the engines have all sorts of built in fixes (eg acceleration control bleed valves) to allow the engine to run at low rpm. Basically a jet engine likes to run near its optimum operating design point, which is somewhere towards the maximum rpm.
You need to consider that a jet is a thrust producing machine while the piston engine is a power producer. For the piston, the propeller takes a bunch of power and converts it rather inefficiently into thrust. The piston/propeller combination is rather complex in that the engine power is largely dictated by cylinder pressures and rpm .. increasing these increases power. However, the propeller has a big problem handling high rpm as the tips rapidly get to local sonic speeds and experience the same sorts of problems which wings do at high subsonic Mach Numbers. As a result you find things like blowers to increase pressures, geared engines to increase engine rpm while keeping the propeller rpm under control, variable pitch propellers to allow a more flexible conversion of power into thrust.
The use of percent power is just a convention .. if it bothers you, you can convert quite simply to BHP or kW according to your preferences .. makes little difference. It is also very useful to get the engine manufacturer's Operator's Manual for the particular engine, as this will give you a whole lot more information than what the airframe manufacturer chooses to tell you in the POH. Much the same sort of thing as CG being quoted in percent MAC .. useful for the aerodynamicists on the project team, but a minor nuisance pain in the neck for the line pilot.
(e) The problem with ill-considered aileron deflection at high speed is that the airflow may accelerate sufficiently to put the local wing section into a shock separation problem area. This is rather different to the more commonly discussed mach tuck problem. I suspect that what you read was more in the nature of a generic discussion point rather than specific guidance.