There are near infinite rituals that can be created, though only 1 in 1000 Normal Rituals typically succeed (due to a lack of knowledge). As of the creation to date, there are only 42 published rituals (specifically normal rituals), all of which are very complex. It should be noted that a ritual can vary in strength, depending on the amount or potency of the magical components sacrificed.
Besides magical components (body parts or objects), rituals also require time and pain. Many rituals take a certain amount of time to take effect, and are typically very painful. Many see it to be a fair trade, as many rituals last for life, unless another ritual is used to remove the given attribute.
There are two other variants of ritualistic magic, hence why the ritual type previously mentioned have been referred to as Normal Rituals. The remaining two ritual forms are known as Blood Rituals and Soul Rituals. This is because they center around the concept mentioned within their names, and work differently than Normal Rituals.
Blood Rituals are based on sacrifice, and are linked via blood (rather than with typical runic magic). One must sacrifice something of near equal value of what they wish to gain, and create a series of runes with their blood.
Soul Rituals are the least understood of the trio. They require no sacrifice, runes, or absorption of any kind. Merely a potent level of intent. They center around the soul, affecting it in some way (perhaps to damage or mend a soul, or to connect two souls as they do during Phoenix Soul Bonds and Ancient Marriage Rituals).'
Harry could definitely see the appeal in such an art. It might be fairly interesting to develop more rituals, potentially giving him more benefits. He would have to make sure he paid attention to anything and everything Salazar said on the subject; he wasn't interested in permanently transforming himself into some unsightly human hybrid, or something equally peculiar.
Besides, from what Harry could tell there were some fairly decent rituals in existence. While he did not truly understand what Soul Rituals were (as the book said they required nothing but direct desire or intent), he could not see how something that brought two people together could be a bad thing.
Salazar had told Harry not to worry about either of the latter two ritual types at the moment. Soul Rituals were mainly used to form bonds with others, something Harry had no need for at the moment. As for Blood Rituals, Salazar said that while they would likely be capable of accomplishing certain things normal ritual could not, they were still not something for Harry to concern himself with just yet. Regardless of how impressive the few Blood Rituals that apparently existed were, they were very few in number, and were typically only used for immensely important occasions.
Salazar had also mentioned a hypothesis he had formed as to how Harry survived being struck with the killing curse all those years ago, one that revolved around rituals. The portrait had initially believed Harry's survival had been due to a Soul Ritual (which would only require potent intent), but eventually decided against it, stating that Soul Rituals could only be used to break, mend, or form bond between souls.
He had therefore changed his hypothesis into a Blood Ritual, and had now firmly stuck to it. Blood Rituals would require a sacrifice, something which could likely have been provided by the deaths of Harry's mother and father. Salazar was unsure of what that meant for Harry, as he claimed it was unlikely such a powerful protection would linger for much longer. Considering that the protection was seemingly against the killing curse, Salazar had advised Harry against putting it to the test.
And when I said I wasn't that stupid, he says you can never tell with a Gryffindor. He's really milking this for what it's worth.
The hypothesis did, however, rely on Lily Potter having knowledge on Blood Magic. There was simply no other way she could have hoped to accomplish what she had, according to Salazar. Harry had known his mother was a prodigy of sorts, but not to that extent; he had always thought the professors had been playing it up, lost in nostalgia. Hearing that Blood Magic was likely the only possible way Harry could have survived, however, did make Harry much more willing to delve into the study of rituals. If his mother deemed it safe enough to learn, he would as well.
Harry closed his book gently twenty minutes later; the Gryffindors' next class, Charms, would be starting shortly. Hermione was reprimanding the other Gryffindor boys again; he would have to remind Ron to refrain from talking about Veela around her.