Where to start learning to understand SQL Server architecture and internals?
Just to add to the suggestions on this thread, you might find these interesting
Books:

I found Thomas Larock's DBA Survior very useful. It doesn't go deep into SQL Server internals, but it has right pointers to guide a newbie towards becoming a successful DBA. If you are past that newbie stage, you can skip this.

Rod Colledge's SQL Server 2008 Administration in action was useful for me as a next step from DBA Survior

Troubleshooting SQL Server by Jonathan Kehayias is a great book. It is available as a free download. Also take a look at other books available on SSC. I am currently reading a book on execution plans by Grant Fritchey.


There are some awesome blogs out there:

Read everything under the category &quot;involuntary DBA&quot; on Paul Randal's blog. While I understand that you obviously are not an involuntary DBA, they tend to be simpler while explaining things to newbies.

Buckwoody has a great blog here and also a useful SQL Server reference guide here. He is blogging about Windows Azure stuff thesedays but you can check the archives


Other Blogs in random order

Aaron Bertrand
Grant Fritchey
Brent Ozar
Thomas LaRock
Pinal Dave
Phil Factor

Podcasts:

There are a few SQL Server related podcasts on DotNetRocks
Also here on SQL Down Under

Other great sites - SSC, MSSQL Tips, SQLCAT, LessThanDot.
This list is in no way complete and if I have missed some important books/sites/blogs, thats because I too am still learning and my knowledge is limited. Lets hope the others on this thread add more resources.
Good luck!
UPDATE: I forgot to include SQL Server Performance Survival Guide technet article which has loads of links and pointers.