To expand on what others have said:
In Book of Hallowed Might, there is an alternate Paladin that gets a few more high-level abilities (including more smites), to make the higher levels mroe fun to play. There are Neutral Good and Chaotic Good variants of the Paladin, which lose a couple Paladin abilities and gain others (can be used with the PHB Paladin or the alternate Paladin). There is also the Dedicated Paladin, which gains Cleric domains (powers and spells) and loses other Paladin abilities.
In Book of the Righteous, you get the Holy Warrior in place of the Paladin. Holy Warriors are dedicated to a specific good deity, and so they may be LG, NG, or CG. The abilities they gain depend on the deity they serve. Each deity has 3 Holy Warrior domains (different from Clerical domains), and Holy Warriors choose 2. So 2 Holy Warriors of the same deity may have different abilities. The spell lists and skills are slightly modified for each deity, and Holy Warriors also gain a specific spell-like ability they can use instead of the Paladin's Remove Disease. Book of the Righteous goes farther than Book of Hallowed Might goes in terms of customizability, and that can be a good or a bad thing. It depends on HOW different you want your new Paladins to be. The Holy Warrior will also take a bit more work on your part if you want to adapt it to other settings, because you now have to decide which domains, spells, and skills are appropriate to each good deity in your pantheon.
Good is apparently similar to Book of Hallowed Might, and has a CG and NG version of the Paladin with different abilities (and names). I haven't seen it, so I can't say more.
Call of Duty is a great Paladin sourcebook PDF, but offers less in terms of customization than the above. It lets you swap out Paladin abilities for others, like giving up spells for more smites. It also presents the Paladin as a PrC option, which none of the above do.
Also, note that Book of Hallowed Might and Call of Duty are currently only available as PDFs, and are much, much cheaper than the Book of the Righteous. I think BoHM is $7, CoD is $5, and BotR is around $40.