A spiritual master is someone in the line of disciples from God, and who has absorbed the knowledge that has been passed down. The Gauḍīya Vaiṣṇava tradition I try to follow is invigorated by the appearance of Śrī Caitanya Mahāprabhu from 1486 to 1534, as He was predicted to in many verses of the Vedas with abundant details. Many of His direct students are spiritual masters, and many people have subsequently dedicated their lives to the profuse teachings coming from Lord Caitanya. This is called the paramparā, disciplic succession.

There is no boundary of service, but devotional service is specifically motivated by love.

On the subject of the oneness of God's names, form, and personality, the best book I know of is Harināma Cintāmaṇi, which is available for free direct download here:
http://ebooks.iskcondesiretree.com/pdf/Gaudiya_Books%20/Bhaktivinoda_Thakura/Bhaktivinoda_Thakura_Hari_Nama_Cintamani.pdf

The Vedas prescribe how to properly learn them, and the most authentic translations I have found are in the books by Śrīla Prabhupāda. The above linked text is written by his guru's guru's guru. Most of what is popularly presented as the teachings of the Vedas is what I consider misinterpretations. The best modern understanding of the Vedas I have found is at www.ashishdalela.com, where I am regularly amazed and inspired.

There are more great books about these subjects than are possible to read in a lifetime.

I somehow previously overlooked your doubt about the legitimacy of referring to the God as a "He." I haven't used this app nearly enough to be proficient. The concept of masculine and feminine in the Vedas relates to enjoyer and enjoyed, and Kṛṣṇa is described as the ultimate enjoyer. The internal nature of Kṛṣṇa is feminine though, as His internal energy is the form of Rādhā, the ultimate feminine person. Kṛṣṇa is identified as God, but He worships Rādhā. Their combined form is Lord Caitanya, mentioned here in the first paragraph. It's also said God is male because the combined form of Rādhā-Kṛṣṇa is a celibate male.

CC Ādi 4.55: "The loving affairs of Śrī Rādhā and Kṛṣṇa are transcendental manifestations of the Lord's internal pleasure-giving potency. Although Rādhā and Kṛṣṇa are one in Their identity, They separated Themselves eternally. Now these two transcendental identities have again united, in the form of Śrī Kṛṣṇa Caitanya. I bow down to Him, who has manifested Himself with the sentiment and complexion of Śrīmatī Rādhārāṇī although He is Kṛṣṇa Himself."
Www.vedabase.com/en/cc/adi/4