True. If you follow LKML, you will notice that the majority of changes are to the device drivers code. And it makes sense, after all, you want the kernel to run on a wide variety of hardware. So, it's not just what fraction of developers work on the core but rather, what the fraction of changes belong to the core kernel - which is relatively small compared to the device drivers code.
Another good way of getting into kernel hacking is not just device driver books but following mailing lists you are interested in. netdev, pci-devel, kvm are some of the interesting ones. Also, checkout GregKH's driver project - these are drivers that are not yet fit of inclusion and hence reside in a "staging" area - a great way to get involved in kernel development ( http://linuxdriverproject.org/mediawiki/index.php/Main_Page). Note that some of the lists can have really high traffic but eventually you will get the hang of picking out the useful stuff. https://lwn.net/Kernel/Patches is a great resource for mention-worthy kernel patches without subscribing to individual lists.
I also agree with you that maintainers will _not_ be cautious and prevent newcomers. That's a bunch of bull. If at all, maintainers will be welcoming to changes from first timers and will thank you for your contributions if your changes do get in. Maintainers or other developers usually are frustrated with other style issues though - for example, lines greater than 80 chars, using a mail client that mangles your patch and so on. But keep in mind that the frustration is on the patch you posted, definitely not on you on a personal level :)
Another good way of getting into kernel hacking is not just device driver books but following mailing lists you are interested in. netdev, pci-devel, kvm are some of the interesting ones. Also, checkout GregKH's driver project - these are drivers that are not yet fit of inclusion and hence reside in a "staging" area - a great way to get involved in kernel development ( http://linuxdriverproject.org/mediawiki/index.php/Main_Page). Note that some of the lists can have really high traffic but eventually you will get the hang of picking out the useful stuff. https://lwn.net/Kernel/Patches is a great resource for mention-worthy kernel patches without subscribing to individual lists.
I also agree with you that maintainers will _not_ be cautious and prevent newcomers. That's a bunch of bull. If at all, maintainers will be welcoming to changes from first timers and will thank you for your contributions if your changes do get in. Maintainers or other developers usually are frustrated with other style issues though - for example, lines greater than 80 chars, using a mail client that mangles your patch and so on. But keep in mind that the frustration is on the patch you posted, definitely not on you on a personal level :)