My first choice would be "Archangels of Magick" by Damon Brand:
Book – PDF - Damon Brand - Archangels of Magick: Rituals for Prosperity, Healing, Love, Wisdom, Divination and Success
The main reason is that invocation (with the big exception of demonolatry) and evocation is usually associated with ceremonial magic with all its elaborate paraphernelia (robes, magic circle, incense, magical weapons like wands, swords, etc.), purifications, endless prayers and fasting and so on, which can be daunting (it sure was for me!). The book uses no such props, is written in a very beginner-friendly style and divided into several levels of difficulty, so you'd start out with simple sigil magic, learn invocation and continue up to evocation.
Disclaimer: I wasn't able to able to get the book to work but don't think it was the book's fault, rather my lack of natural talent and preparation (and by 'preparation' I mean groundwork like meditation, visualisation, breathwork and energy work.). My main beef with the Gallery of Magick books is that they shrug off the importance of preparatory work on oneself (which can easily take two years or more) and make things seem deceptively easy - and it
can be easy provided you're naturally gifted. In Damon Brand's defence it must be said that he's a little bit more thorough with invocation where he suggests a (very brief) period of time where you try to get familiar with the archangel you plan to invoke, something E.A. Koetting calls 'preparatory immersion' or Tibetan Buddhist monks the 'generation stage', so it's not the usual GoM "quick n' easy" approach.
Another recommendation would be to try "The Bornless Rite" on p. 85 in Jason Miller's "Real Sorcery" (I would also recommend the book for its groundwork program):
Book – PDF - Real Sorcery: Strategies for Powerful Magick - Jason Miller (new edition of sorcerer’s secrets)
It's an example for my opinion that invokation is a relatively new method pioneered by the Golden Dawn because historically, it was either full possession or the assumption of godforms (as in the "Bornless One" lifted from the Greek Magical Papyri, ca. 3rd to 6th centure CE, doubtless more knowledgeable members will be quick to disprove my theory). In this ritual, you don't invoke a god but
become one. The Bornless Rite is a bit of a lithmus test, IMHO - if it doesn't give you goosebumps, you're not ready yet, but that's my personal opinion.
As for theory… my opinion is that you can easily get lost there. Ancient magic was mainly about contacting the spirits, so you can study paleolithic Shamanism, Sumerian demonology, Egyptian magical practices, the old medieval and Rennaissance grimoires, modern psychological theories about spirits and wade through contemporary quantum mechanics psychobabble for decades. Just about every book on magic contains chapters on spirits; reading a lot of background information, however, can be both illuminating as well as a way of procrastinating, so it's basically your choice what and how much you read.
Every author and every (aspiring) magician has their own view about spirits. What's more, Tanya M. Luhrmann describes in her book "Persuasions of the Witch's Craft: Ritual Magic in Contemporary England" (a scholarly anthropological study, also in the Library) how most magicians go through several interpretative shifts in their careers depending on their experiences so their views are likely to change over time. Anyway, personally I find it important to avoid ascribing human psychological patterns of behaviour to beings that are basically ineffable; one example would be prayers and offerings - they are not means to appease, pay or bribe an entity but rather ways to attune
oneself to them. This opinion of mine is also subject to change pending actual personal experiences, of course.
Phew, long-ass post. I think I've just about covered it. You will find tons of books in the Library when you search for "spirits", so I'd better shut up and leave you to your studies.