Not sure what you're looking for, so I'm going to be pretty scattershot....
Smokey Yunick's Power Secrets is often recommended as a place to start.
The Horsepower Chain - Racing Engines Optimized Through Hardcore Math sounds promising.
Engine Builder magazine might be useful.
Hot Rod's Engine How Tos are pretty practical.
I already know the basics, so it would be a matter of tuning for me, and Dyno and Engine Simulation software would likely be useful.
I found Epi-Eng's Formula 1 vs. Nascar article highly informative. Epi-Eng has a lot of good info!

RodRico thanks for the info, I read the Epi-Eng article; that is exactly the kind of thing i'm looking for thanks. I will look into he other stuff to given time.

"Design and Simulation of Four Stroke Engines." By Prof. Gordon P Blair.

je suis charlie

This may be more historical than what you are looking for but I found it a good read:

Classic Motorcycle Race Engines: Expert Technical Analysis of the World's Great Power Units by Kevin Cameron

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

You can find a decent albeit basic overview of the technology used in each series online via google. Beyond that, I wouldn't expect to find much as racing today is more about tweaking a series-approved standardized box than it is new technology. Not to knock it in the least, but promoters and fans alike prefer teams to be competitive without any obvious advantages which has led to rules preventing too much technology from filtering in. Regarding books and publications, I prefer history and hot rods for the entertainment value as racing pubs like racing trade-shows tend to be mostly focused on selling parts rather than sharing useful info. If curious to dig deeper, I'd recommend "Race Engine Technology" magazine and/or a trip to the PRI show in Indianapolis.

If you're genuinely interested in modern engine performance, the only real way to learn is by going to work for a major OE. Whether its power density or fuel economy, everything today is about efficiency which requires a high level of competency by a team of specialists ranging from combustion simulation to component design. Recip engine design is a massively broad topic that could easily fill multiple careers. Very important to performance gurus, its also full of contradictory trends and exceptions to those trends so performance can be rather tough to predict accurately based on governing equations and generalized trends without a ton of testing and experience. That's not to say there aren't a ton of folks with very limited knowledge and experience hocking software and "expertise," most males think they know something about engine performance yet one truth that has stuck with me for ten years in this niche is a mentor's words: "forget everything you think you know about engines."

Thanks guys some more good leads to explore.

CWB1; I have worked in the Power / Oil & Gas industries since being an apprentice, I'm now 37 and although I have a Mechanical Engineering degree the career change would be a bit challenging at this point, but it would be an interesting job. What would be required on top of a B-Eng-Hon's Degree in Mechanical Engineering?

I live in the UK by the way, and I would love to have a job involved in motor racing / mechanical engineering in some capacity..

Unless your experience in applicable to motorsports, your best bet would likely be to take a couple small steps. Leverage your experience in oil & gas to get into either field service engineering, application engineering, or manufacturing for one of the diesel engine OEs that serves the oil & gas sector. After a few years jump into the design office. After a couple years of that jump into motorsports.