Where can I read more about Proto Slavic?

Step 1: Trust the OCS. If you haven't, read the Wikipedia article: https://en.wikipedia.org/wiki/Old_Church_Slavonic . OCS is the first written Slavic language and was intelligible to speakers of other branches of Slavic languages descended from Old Slavic, which strongly implies that a speaker of OCS, blessed with a makhina vrjæmene (time machine), could have communicated with a speaker of OS. Read the Lord's Prayer at the bottom of the article (I had this memorized once) looking up IPA if you don't know how it works yet. Many people treat OCS as a stand-in for OS, even though they know there were differences, because of this mutual intelligibility.

Step 2: Listen to some stuff. This sounds right, but I can't see the text. I can hear all the syllables though: https://www.youtube.com/watch?v=4w0KUQjGRh4 .

This is awesome: https://www.youtube.com/watch?v=o_ZSoeWT1FU !

This on the other hand is a false-claim OCS song, actually in Old Russian (maybe): https://www.youtube.com/watch?v=3EQynq1GcKw . Notice the difference.

This has all the syllables, but with Russian pronunciation, analogous to the way that "Church Latin" is pronounced as if it were Italian: https://www.youtube.com/watch?v=a8zzQOlZGXQ . The fault there is that Ѫ Ѭ Ѧ Ѩ , were nasalized, and the speaker has dropped that. They were, in order, approximately like on, yon, en, and *yen would be in Japanese (I don't think Japanese distinguished the last two anymore, and neither does Russian, which has taken these to o, jo, e, and je.) There are a lot of videos with the same guy, including some comparisons to reconstructed Old Slavic.

Step 3: If you want to dig, and read cyrillic, and are willing to fight a very terrible rendition of cyrillic, you can go after this resource: https://lrc.la.utexas.edu/eieol_toc/ocsol . This is part of a project to put IE linguistic material online, by the University of Texas at Austin (where I studied OCS ages ago). Knowing Cyrillic beforehand get around the fact that some letters are rendered at a different size than others. Of course, that might be computer dependent. The site has alphabet, grammar, and readings, arranged in the order of a text.

Step 4: go after scholarly stuff on Old Slavic and push the rest of the way back. It'll be like pushing from Imperial Latin back to c. 400 BC Latin though, really close.

Good luck.

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There are some sources you can go to. You have to be aware of two major factors, though:

a) there are no introductions to Common Slavic for laymen
b) most of the material on CS and OCS is in German, French or a Slavic language

Okay. Now that that's out of the way, let's go...

OCS is considered to be latest developmental stage of CS, a southern dialect of CS. As such, OCS opens up a door into CS, but they are in no way identical. This is due to the fact that while OCS is based on the dialect of Soluń (Thessaloniki), it was codified in Moravia and shows influences by West Slavic and Old High German (unlike generally thought, many terms of christianity were either directly loaned from OHG or coined after them). Another point to consider is the fact that the OCS we know isn't the earliest stage of Church Slavonic, either. That earliest stage, called Urkirchenslavisch in German, was the language that Cyrill and Method translated into, but unfortunately all sources from those days were lost in time. The structure of the Glagolitic alphabet (the original alphabet invented especially for OCS) tells us, that certain sound changes took place. Some morphological variants on the other hand tell us that Proto-ChS (and therefore CS) had suffixes in use that were lost (like e.g. a genitive in -s, as can be seen by the unique case of чесо (instead of the expected чего), the genitive form of the pronoun чьто 'what' or root-aorists).

For OCS, there's a shitton of sources around. The best sources available are the Handbuch des Altbulgarischen by August Leskien, which was (and still is) used by generations of scholars and students of IE linguistics. Another very good source is Старославянский язык I + II by Selishchev, which has a major focus on phonology and A. Vaillant's Manuel de vieux slave. Also, the Altslavische Grammatik by H.H. Bielfeldt is VERY well worth searching out, as it's not only a historical grammar of OCS, it also consequently compares OCS structures to Modern Russian. The most inevitable sources are the Altkirchenslavische Grammatik by Paul Diels and Le slave commun by A. Meillet, as they are the only grammars of OCS that lists ALL morphological variants that can be found in the canonical texts. In English, there are only three works on OCS I can think of - the short, superficial and quite lackluster Old Church Slavonic Grammar. An Elementary Grammar. by S.C. Gardiner, the Old Church Slavonic Grammar by H. Lunt (which I don't like and use because of its purely synchronic view on OCS), and the Old Church Slavonic Grammar by Nandris, which is hands-down the best grammar of OCS there is in English.

For Common Slavic, the situation looks much bleaker. There are some works that cover aspects of CS (Common Slavic Nominal Morphology. A New Synthesis. by Orr for nominal morphology comes to mind, as does The Prehistory of Slavic by Shevelev for phonology), but complete grammars of CS are exceedingly rare. The only two works I can think of are the Urslavische Grammatik by J.J. Mikkola which is quite short and was published in three separate volumes (146, 57, 106 pp) between 1913 and 1950 (the oldest part is the volume on vocalism which is terribly out of date by now) and the equally named Urslavische Grammatik by P. Arumaa, published between 1964 and 1985; once again, the oldest part is on vocalism, but it's still usable). Meillet's Le slave commun can be considered a grammar of CS as well, but as the second edition was published in 1934, it's quite out of date as well. The most modern work in this list is R. Večerka's Staroslověnština v kontextu slovanských jazyků (2006), which is not a full-blown grammar of CS / OCS, yet does a remarkably well job.

Perhaps you need to search in Russian language.