I'd actually suggest starting with the LBRP & Lesser Hexagram ritual myself. The Star Ruby is great but you won't understand the inner workings well unless you work a little bit at least with the LBRP. The Star Ruby involves much more complicated symbolism than the LBRP, they aren't interchangeable like some think. I used to think that way and in the lat few months I've really been exploring both rituals and they become more & more apparent as I research and work with them. The lesser Hexagram is an excellent & important ritual that should be in the repetoire of every magician. This ritual invokes or banishes the four elements and helps equilibriate the pentagram rituals & again, understanding the original form will enhance & enlighten you on the Star Sapphire, Crowley's Thelemic version.
As to LIber V. I never performed the GRP before when I learned Liber V and regret that immensely as so much of the ritual relies on having a grip on the 5 forms of the Pentagram and being able to control those energies. Learn the GRP before launching full scale into Liber V, work with the GRP for awhile actually as this ritual is extremely important to a LOT of later work in the area of invocation & evocation as you learn to work with the ritual & unlock more & more of its secrets. SO many rituals use this one as the underlying framework like Regardie's Opening of the Watchtowers & as I said, Liber V.
Liber Resh is simply the BEST ritual to start with as it is simple & easy to memorize. It also helps in maintaining discipline as you work to keep the ritual up from day to day & it is easy to do in public such as if you are at work, either excuse yourself to the bathroom for a minute or recite it under your breath. The ritual opens up a lot of energy & even helps in maintaining a sense of time & even place.
The best book you can get on the subject is Lon Duquette's The Magick of Aleister Crowley as he goes over every ritual in Liber 0 & explains them in a clear manner including some elements that aren't readily apparent. Liber ABA is a great book but not for the beginner in spite of AC's efforts & belief to the contrary. Duquette's book also has the added benefit of discussing other materials that appears complex in Crowley's work but on final examination he reveals how deceptively simple it really is & presents some of them in a more workable manner such as Liber Samekh. The chapter on the Gnostic Mass is excellent & before having read it years ago I was not a fan of the Mass. After reading Lon's commentary on the Mass I had a much greater appreciation & especially after finally experiencing it. Lon's introductions to the material are first rate and my only complaint is that he overly complicates Liber V in his discussion of the "Averse Pentagram" and what Crowley meant by it. This book will answer most of your questions if not all of them.