While I generally chime in with Alan's later message, one important you should start reading about and differentiating 

is Authentication and Authorization (the later is Accounting of AAA with RADIUS). 

While you can do Authorization using LDAP with AD, you can't do the Authentication part using LDAP against AD.

Using Samba and ntlm_auth is the way to go, that due to to how AD stores passwords.

Read deployingradius.com, specially the compatibility matrix and "Authentication Systems and Password Compatibility". 

You may do LDAP load balancing on the authorization part, but ntlm_auth and balancing / failover is done by Samba.

Otherwise if you want to go deeper, get a RADIUS book :-) I can confirm that the initial curve may be a bit steep if you

haven't done any RADIUS before, but it's well worth since it gets you better overall understanding  on AAA and RADIUS, that will 

definitely help if something goes belly up.

-- Mathieu