The Book of Common Prayer

No, not that one, but inspired by it certainly.

One of the greatest advantages of the cleric class (if not the single greatest advantage) is their ability to gain any spell in the cleric spell list without having to resort to spell books, scrolls, or other "memorization media." This means that every cleric - assuming he has access to the appropriate sphere and level - can cast a healing spell, and the prayer spell, and all the rest, which is a startling advantage over magic users who must build their inventory of spells over time as they adventure. Their only limiters on spells are the requirements for rest every day and daily devotions in order to regain spells. It's also presumed that the cleric is required to further the cause of his ethos/god or otherwise stay true to his supposed religion, but this is ignored frequently enough that it might as well be optional.

Whether or not this disparity is "fair" is not my concern here. What's more interesting to me is the number of minor problems this issue can introduce.

First, it leads to a major issue if the DM wants to introduce new spells for the cleric from other sources, especially midway through a campaign. This can get especially dangerous if the ref isn't careful and doesn't make sure that the floodgates stay closed, thereby letting in a flood of spells into the game that he didn't want to begin with. In all fairness, while it might be great fun for me as the DM to throw a cleric with the "Spittle" spell at the group, it's not wonderful to then turn arond and tell the party clerics that not only can't they have such a spell (for it is evil in its own way) but that they can't go outside the bounds of the PHB to begin with.

Another major issue as I see it is that it leads to a disconnect between the cleric character and any possible organization of the religion he serves - i.e., the tribal teachings of the shaman, or "the church" as it were for geographically broader faiths. Just what attachment does an adventuring cleric of Pelor have with the church of Pelor anyway? What are his duties to the hierarchy if any? What penalties will he suffer for failing to oblige by them?

There's also the issue of holidays, fasting days, feasting days, and the religious calendar at large, though that is entirely the province of the DM and unique to each campaign.

My solution is the introduction of a holy cannon for each sort of priest in the campaign. Each individual religion will have its own holy texts (i.e., its own bible) which all of their clergy will be required to be familiar with. In it are the teachings and stories of the god, the history of the church if appropriate, and tellings of the miracles performed by the faithful. For the spellcasting members of the clergy (i.e., those with PC classes in Cleric, Druid, or any other class that features spell casting) these texts serve as essential tools without which they find the performance of miracles increasingly difficult: for every 24 hours period without their cannon, a cleric/druid character faces a cumulative 5% chance increased spell failure chance. These texts contain all of the spells available to cleric characters out of the PHB for the first three levels. At such time as a cleric character would be able to cast 4th level spells, he must make a pilgrimage to a suitable stronghold of his faith and present himself to the authorities there to make a reckoning of his career thus far and plead his case for access to the greater mysteries of the faith. Only if the authorities (an abbot or high priest or whatnot) are satisfied by the accounting will the priest be granted access to more complete sacred texts and permitted to copy from them. The whole process requires a week per spell level gained (i.e., gaining access to 4th level spells requires 4 full weeks of prayer and copying. Thus, replacing a lost or stolen cannon will require a significant investment in time and effort.

A priest may, of course, add spells gained upon his travels to his cannon, but such may cause difficulties when returning to the church for reckoning as spells outside of the cannon may be considered blasphemous. Unscrupulous priests may even pay for completed texts rather than subject themselves to the scrutiny of their superiors.